Background.The Knight Errant is said to be spiteful of spirit due to the bloody nature of these marks of armour that survive long. Only the most iron willed commander being able to truly master them and survive long inside them. On the battlefield, they're basically designed to take out enemy super heavies.
These mark of knights are very similar to the Knight Paladin builds except instead of the battle cannon, they utilise a thermal cannon. This has one less shot, but provides the melta rule. Importantly, this has an AP of 1 which makes it deadly to anything in the game. Even primarchs should have cause to fear a knight errant when they see one.
To be clear, these guys are dedicated to taking out tanks, super heavies and high armour infantry targets like terminators.
One should recall at all times that many (all? because of game theory) tanks have armoured ceramite. I've actually written about why I favour melta weapons in 30k before on the grounds that they're not that common because the assumption is that everyone will have armoured ceramite. That may sound contradictory, so go read my article on it for the full background on my views!
As with the Knight Paladin, I'm unconvinced that the bio-corrosive rounds are really required. And the ocular augmetics are a bit situational with respect to night vision. But here, with the melta rule in play, they could be a huge boon.
The only real question here to my mind is whether to go for the ocular upgrades or not. I think its probably just about worth it here given the melta weaponry. Hence my suggested build is as follows.
Knight Errant, Occular Augmetics (380 points)
The household ranks that might be favoured here include the HQ selections (both Seneschel and Lord Scion) due to bringing the fight close up, as well as Aucteller in the Elites section (think: outflanking).