The Knights-Errant are a unique force in the Horus Heresy; assembled by Malcador the Sigillite and assigned only to the most critical battles as their loss would be huge for the Imperium at large. As such, their inclusion in any battle will dramatically change the rules for victory.
A Knights-Errant must select from one of a short list of Oaths. Some are good, some are easy to obtain, and others as tough as anything.
There's a few catches. Firstly, the selection must be done in secret and a note made. Only at the end of the game is this note revealed when tallying up victory points.
However, if the Oath is not achieved then the side fielding the Knights-Errant may not win at all. They can only draw at best! Hence, there's an explicit issue with fielding a Knights-Errant -- choose wisely!
Crusader is the easiest to achieve and has the smallest points value -- being in the opposition's deployment zone will be rather easy for the Knights-Errant given their use of the Falsehood teleportation system if desired. This should be the default choice for the less adventurous or those just starting out with Knights-Errant.
Protector is slightly tougher: a given other independent character must survive. Now, this is an entertaining option as the temptation will be to put the Knights-Errant inside the same unit as the selected character and take the wounds for them. Alternatively, sequestering them in a land raider and avoiding combat is another nice way to go about it. This could be a very easy Oath to fulfil, or could be a nightmare. It just all depends on the opposition really.
Hand of the Sigillite is a nice option for those playing objective games. Having control of one objective should be straight forward if played well. Having that objective in the enemy zone is tougher but not impossible given deep striking accuracy and a slower moving enemy. Select this one carefully. It could be easy, or it could prove impossible if facing off against a fast moving army containing many bikes or assault squads.
Headsman is tricky. Slaying the enemy warlord in a challenge is tough. Not only does it require the meeting of your Knights-Errant with the enemy warlord on the tabletop, but a victory as well. Now, in a challenge, the Knights-Errant are usually going to do well, but that obviously depends on which one you are fielding and how the enemy warlord is kitted out. But if the enemy warlord has alternative plans, then this could be a huge risk and gamble. I'd only take it if I knew I could get in to a challenge with an opponent's warlord. And that would usually be done with my friends by jovially winding them up to face such a challenge. But they're smarter than that and would see the ruse!
King Slayer is the hardest of the lot in some ways. Slaying a Primarch is a big ask under any circumstances. And if the opponent is fielding a Primarch, then you might be as well. This could well lead to a stale mate. On the plus side, the Knights-Errant doesn't have to do this personally like the above Headsman entry. The enemy Primarch just has to die. I can't see Horus going down easily to be honest. And even the weakest of the traitor Primarchs, Alpharius (or Lorgar, depending on whether he's ascended or not), are a tough ask to deal with and can survive many things in a 6 turn game. Beware of this one. Take it as a fluffy choice, and the right thing to do!