Synergy is not required - just brute strength
This was one of the original alternate wincons that felt doable in most Magic formats, though its benefitted from an influx of extra gate cards in various MTG sets, which means you dont have to be a 5-color deck to pull off a Mazes End victory anymore
If the creature this Aura would enchant is an illegal target by the time Knightly Valor tries to resolve, the Aura spell doesn't resolve
While none of those effects are particularly enticing, the sheer number of options makes any card look good
Do note that in a casual game, not only does the four copy limit not apply to Legion Angel (except within your deck, as normal) and nor are there any rules as to where the copy outside of the game comes from meaning you can just pull it from your personal collection of cards, as long as you have them with you), in a Tournament there are restrictions on how Legion Angel works