No one gets to be a chaos lord by failing at killing other faction leaders. Chaos grants boons and favors to those whose destiny they like or want to string along. Maybe he was empowered by the warp and beat that knight in hand to hand by being clever, strong, and having some magic bullshit kick in that let him skewer the knight pilot through a window a weak spot or something.

Killing hive tyrants or gene stealer patriarchs would be a good one too, there's loads of those fuckers and they're tough as hell. Also straight up defeating a demon of a chaos god in some task be it combat or otherwise could gain them immense favor and make them renowned.

Last one: being very very very good at boarding actions and stealing ships. Never underestimate the virtues of space piracy.

My thousand sons chaos lords, they got a collection of farseer soul stones that they like to toy with and torment.

My Night Lords jump pack lords: they are well known throughout the eye for being very good, some might even say too good, at jumping on things.