looks good, I can't say much about wether it fis belial's lore or not as I'm not familliar with diablo.
here are some suggestions I have:
- his trait could also be that when his health drops low or belial takes a big amout of damage over a short duration he creates copies of himself to confuse his enemies
- the Q is essentially Sylvanas' Banshees. Maybe you can come up with something more unique?
- Wall of lies sounds really fun^^, so one of the belials surrounding the target is the real one?
- Dark Promise is a bit odd, yes the target is hostile with his teammates but they do not need to deal damage to each other, means the ability is very easy to play around. Maybe make it so allies around the target are forced to attack him for the duration? similar to Winter Wyvern's ultimate in dota.