This edition of 40k is the most permissive edition of 40k that I personally think they've ever made. I got into 40k during the second edition era and while I didn't play a ton of games before 3rd edition came around, it certainly didn't let you do some of the combos that you can do now. This is a good and bad thing for people who want to play a single "pure" force. The benefit you have is the ability to take multiple detachments of the same force (for example a primary tau with a tau detachment) in order to really stock up on your strong units if you want/need to while sticking with a purist mind set. The down side is that your opponent can much make up for any deficiency his main detachment might have by bringing whatever they need from another army, or just making a really crazy combo/deathstar. What does that mean for an all knight army? It means you're going to have to worry about a lot more than you would have in past editions.
The knight paladin is the knight with the 2 shot battle cannon and 2 heavy stubbers. This guy was really good, but in 7th he has lost a bit of his pep. The battle cannon only being ap3 really hurts his ability to be any kind of a reliable vehicle killer. Yes he does get 2 large blast ordinance shots, meaning you are more than likely going to pen unless its armor 14, but you will never 1 shot a vehicle that isn't open topped. They also struggle against anything with a 2+ armor save. Sadly, most of the things in 40k that have a 2+ armor save are a threat to your knight in hand to hand. The 2 heavy stubbers are nice, but they are not really great at anything more than giving you options for the assault phase. Being able to fire at up to 3 different targets give the paladin 3 options in the assault phase. In practice though I've personally found that you'll fire both heavy stubbers at the same unit because the strength and ap on them isn't good enough to really do a lot of damage.
Now that I've given the bad news for the paladin let me say that he really is needed in an all knight list. Your weight of fire without him is going to suffer very badly. You need this knight against just about any army you're going to face. Without him you will never kill your opponent fast enough to win. He may not be able to instant kill terminators, or 1 shot every vehicle, but he can quickly clear out pockets of resistance just due to the 10" wide path of destruction that battle cannon can potentially lay down. One very important thing I've learned about 40k is that normally the more dice you get to roll the better from a shooting stand point. This guy will give you more dice to roll in the shooting phase.
The knight errant is the knight with the giant multi melta gun called a thermal cannon. Thanks mainly to the thermal cannon, in 7th edition this guy is really good. The thermal cannon is 1 strength higher than a multi melta, is 36" range with melta effect at 18", and uses the large blast template. It only has 1 heavy stubber instead of 2, meaning you can't give yourself the option of potentially charging 3 different units, but this isn't really a big deal. The bad news for this guy is pretty much that he only gets 1 shot with the thermal cannon. This isn't the end of the world, but it does make you want to be more selective with your target choice.
If the paladin is the grunt of your knight army, the errant is the sniper. He only gets 1 shot, (stubber notwithstanding) but whatever he picks is likely dead. In 7th anything ap1 is now amazing. This is why this knight got even better in this edition. Granted in 6th he was just as good, but the change to the vehicle damage chart puts this guy in rare company. Not many guns in the world of 40k are ap1 let alone large blast and melta. This is your 2+ armor save killer and your land raider death machine. He has a 40% chance of exploding any non super heavy in the game, and a 60% change of doing it to open topped vehicles. You need this knight to clear out the really big nasty stuff.
The cerastus knight lancer is the new knight put out by forge world. This knight has a shock lance as its 1 and only weapon. In 7th edition this knight is very so so, borderline not usable for truly competitive play. The shock lance is both a hand to hand weapon and a ranged weapon. The rules can be found for it on the forge world website or in the horus heresy book III, but I will go over them here since the rules are currently freely available. For shooting the lance is range 18" strength 7 ap2 heavy 6 and concussive. The fact that it gets 6 shots is nice, the strength/ap is good, but the range just hurts it a ton. If it were a longer range you could argue this knight could work as an anti air unit, but the 18" just kills that idea. The fact that if you shoot at a flyer also takes away your ability to charge anything also takes that away. The melee stats on lance make it strength d and you get +1 initiative on the turn you charge which is pretty good. The biggest problem the lancer has is that its 400 points with the ability to only kill or break 1 unit a turn. The paladin and errant can hurt multiple units a turn and are cheaper.
So why take a lancer? If you like using knights in hand to hand, this is the knight for you. They will do a good job of taking care of some of the other things your knights might worry about. If you have a daemon prince you don't want your other knights to get stuck in hand to hand with, or maybe something else with a 2+ save that you can't devote the fire power to, the lancer can potentially take care of that for you. The 3d6 run move means they can in theory get across the board fast to help accomplish that. You do need to make sure that you're the one doing the charging with this guy so that you're not wasting that lance ability to make him +1 initiative. The lancer also has 4 base attacks instead of 3 and has a 5+ invulnerable save in hand to hand which the other knights lack. The lancer is a solid unit but not a must have. In a tournament environment where you don't know what to expect, he probably won't make his points back for you. As a casual game where you at least have some idea of what to expect I can see a potential use for him.
As an additional note I have used the lancers, I've used them against other super heavies and some big nasty named super daemons. Against other heavies they did fine but were unfortunate to get caught in the death explosion and die themselves. Against the super daemons they did not do so well because the daemons were either a higher initiative or had a weapon skill so much higher than mine I was only hitting on a 5+. They did however still kill their target when they blew up, although it is a terrible idea to simple hope your knight takes your opponents with them. Yes you get revenge, but in a all knight list you need every knight to pull his weight. Against normal stuff, the lancers should do well with the higher initiative, +1 attack, and invulnerable save in hand to hand.
The last knight you need to know about is a knight special character. Gerantius the forgotton knight is a knight errant on steroids. His rules were printed in the white dwarf weekly magazine issue 24. He follows the rules for the knight errant, so thermal cannon yay, plus he has it will not die, +1 to his WS/BS, his ion shield save becomes a 3+ instead of 4+, he can shoot and still run after shooting, and lastly he gets to reroll results of 1 on the stomp table. In case you didn't notice I try to talk about the bad first, but there is no bad with him other than not being able to have more than 1 of him and maybe that he costs 500 points. The points thing though is highly debatable.
If you're running an all knight list, there is absolutely no reason why he should not be in your list. Gerantius is an auto include in any all knight army. Your opponent will never be able to kill this guy baring bad rolls on your part. You can always run which means he should never get stuck in hand to hand and the it will not die rule coupled with the 3+ invulnerable means the few hits that do get through from shooting will potentially go away. The only way you don't use this guy is if you don't like or can't use special characters.
How do you put these guys together to form a single awesome army now? Well first 1 knight gets to be your warlord. The warlord knight gets +1 to his WS/BS and his invulnerable save increased to 3+ on the ion shield. He also gets to roll on the special warlord trait chart for knights which has some really good stuff on it, meaning you should always roll on that chart and not on any of the ones in the rulebook. I have tried every knight as the warlord and I've personally found the errant to be my preference because I like making sure that melta blast isn't going to scatter too far from my target, but I will say the lancer potentially would benefit the most from all the bonus things the warlord gets. That isn't to say he should be your warlord, its just that he gets the biggest buff from it in his specific role. The better ballistic skill and weapon skill means those 6 shots are likely all going to hit and in hand to hand against most things you will not hit on a 3+ instead of 4 with an initiative of 5 on the charge. That helps the lancer so much and might make him worth all 400 points. Also 4, arguably 5, of the warlord traits you may get make the lancer better. One of them gives him +1 to charge and run distances as well as knights within 12", one makes enemies within 12" use their lowest leadership value instead of highest which could help fear work better, one lets him reroll failed to hit rolls on the turn he charges (amazing for the lancer), one gives him outflank, one gives +1 to building damage rolls, and the last gives him it will not die.
The only reason I can find for making a paladin knight your warlord would be because you plan on just having him sit back and never get shot at. The 72" range gun means you can put him anywhere pretty much and keep him out of direct danger so you don't give up slay the warlord. Slay the warlord would be a good point for not making a lancer your warlord since he is likely to be out front and taking shots, but again the lancer can get really good.
Once you have the warlord picked you are ready to play (I have not used the optional knight ranks as I prefer to take as much randomness as possible out of 40k so I will not go over these). The things you need to really focus first on are units with objective secured (you don't have it), flyers, deep strikers, and anything that can kill you in hand to hand. Shooting attacks, other than from flyers, are never going to be a huge issue. Your opponent is going to have to throw a ton of shots a a single knight to kill it. In early turns your ion shield will take care of quiet a few shots right off the bat. Armor 13 on the front coupled with the change to the vehicle damage chart means the few shots that do get through are very unlikely to ever take off more than 1 hull point.
Flyers are a threat because in a pure knight list, beyond the lancer, you have no answer for flyers. They will just keep buzzing around and shooting you. The meta where I play doesn't have many flyers, but if you run into a cron air player he will probably hurt you badly. If this happens hopefully you have a lancer that can get into that 18" range. Its not the most effective use of a 400 point model but it will help. Helldrakes are not a big deal for you, the tau flyers are not too big of a deal and most tau players don't use them, all the other armies flyers can potentially hurt you badly but they shouldn't be in large enough numbers most of the time to matter a whole lot. Just be aware that you may have an issue with flyers, particularly thanks to a certain formation.If you can move in a way that makes your opponent either fly off the board or drop into hover mode, provided the flyer can. The limited pivot ability of most flyers and your 12" move should make this somewhat possible.
Deep striking units can become an issue for grabbing objectives, or for making you move that ion shield around. There isn't a whole lot you can do about either of those issues. When possible make it as hard as possible for your opponent to get shots at you from multiple arcs. I know its easier said than done, but just prioritize the threats. If something deep strikes and you have to worry about shots from various facings pick the side that has the most potential damage and hope your opponent rolls poorly. You can also position your knights to make your opponent split up his firing. Knights take a lot of shots before they go down. If you can make your opponent split up his fire among 2 or more knights that's less chance he takes one down. It make sound crazy but sometimes showing that back armor on a knight (try and have a tactical reason for this so it isn't super obvious what you're doing) can make your opponent split the fire of his army.
The things that can kill you in hand to hand. This is such a huge thing to worry about. Knights do have d weapons in hand to hand and they do get d3 stomps, this does not mean they are gods in hand to hand. A normal knight has 3 attacks at WS4. The best you can ever hope for is hitting on a 3+. You will likely need 4's to hit more often than not though. So 2 hits is the best you will ever likely get on average. That's 2 dead guys in a unit, then your opponent gets to hit back. If you're playing orks, that's a lot of ablative wounds before you kill the 1 jerk with a power claw. If you survive the hits against you, you then get the stomps. They are nice, but not a guarantee to save you from the daemon prince you failed to kill with your d hits. You also don't want to get stuck in hand to hand with some blob unit that will tie you up for multiple turns. You probably only have 5 knights and you need them shooting or getting to objectives. Hand to hand is the biggest weakness for knights. You don't have enough attacks to realistically kill anything bigger than a small squad. Your best bet with a big squad is to hope you stomp the heck out of them, or they lose combat and run. Knights are great at killing vehicles in hand to hand, to include super heavies if you're willing to risk the potential d strength resulting explosion, but not so great against infantry. So try to avoid being stuck in assaults that aren't of your choosing.
The last thing is the most critical in most games of 40k. Kill things with objective secured. With 5 knights and maybe 4 objectives to take, odds are you won't be able to cover them all. Your opponent most likely will. The objectives you can't take, you need to at least make sure you can shoot the guys that might be camped on them. Short of plague marines and normal marines, you have the fire power to force those moral checks. If you force enough of them, your opponent will fail them. For holding your own objectives you have to make sure things like eldar jetbikes can't swoop in a take it from you on the last turn. That isn't easy by any means, but sometimes you may have to ignore a target to shoot at some jetbikes. If shooting them isn't an option make sure that you're on the objective and can charge if they get close to it. Going second and making sure you have the last turn isn't such a bad thing for knights. That can really save a game if your opponent tries to swoop in for the objective you're standing on. He swoops in, you shoot him/charge him and boom they don't get that objective. In every game I've played with an all knight list I've gotten first blood. If you keep those objectives clear and didn't lose your warlord or give up line breaker, then you win just by default. Even going second the odds of losing a knight turn 1 have to be astronomical in this edition. Use that to your advantage and just choose to go second when playing an objective based mission. If its a kill points mission you should pretty much always win unless you get tabled.
I hope this has given you some insight into how to play an all knight list. It is by no means an auto win or easy list. Feel free to leave comments on what you think or ask questions. My hope is that forge world really helps flesh out the knights some. They have at least 2 more knight types coming out that look to be true infantry killers. When those come out I will make sure to post another blog about them as well after I get some games in with them.