Bis demon for abyss lock? All I hear is hephasto for echo build but what about abyss?

Cheers

no auras or curses really matter and you can't get very relevant DPS from your demon because you're +chaos skills and can't spare the skill points
having cursed still helps your mercenary though, same with fanat aura.

Hephasto has a huge hp pool but no regen or abilities, and he's easy for you to get aura enchanted. But if you had your pick, an urdar is better with stun attack and massive regeneration and 75% dr in A5 frigid/acheron.
But since its low damage, you're probably better off with a vile mother who will spawn 5 vile children as meat shields, then she'll still chip in the same cursed hits and provide aura. Just not easily found