This spell summons a Felguard as a pet, and it costs a Soul Shard. It can be obtained by investing 41 talents into the Demonology tree. It is like a combination of a succubus and a voidwalker.
Passive ability, the Felguard's damaging melee attacks cause a demonic frenzy, increasing attack power by 5% for 10 sec. This effect can stack up to 10 times.