Sion couldn't revive against a nightsister team

So I just battled a nightsister squad with my sith team. Sion used Held By Hatred followed up by Torment and got stunned after that. When the nightsisters killed Sion a few turns later he just died and didn't come back to life.

Do nightsisters have a "prevent revive" thing somewhere in their kit? Because I can't find it. FYI Sion got killed by Ventress her Endless Wrath abilitie.

Their team was: Talzin(L), Daka, Ventress, Spirit & Zombie.