How come do you think he has only collected certain monsters for his plan? He so far has the sphinx and pothos but there are other monsters who remain intact i believe in their dead bodies and would be a waste to let them decompose
The Necromancer? No, Malivore sends out the monster. Although with Kai it seemed more as a transition between dimensions. They didn't really explain that but the former is likely if another monster pops out before Kai's death/or banishment.
@Bennet Blood Mage18 i think Kai came out so fast because he's a vampire and a witch so Malivore holding him would probably hurt it cause we already know Malivore can't hold vamps,witches and werewolves for long.