death's door

you send a target to a pocket dimension with 4 doors. the person makes a wisdom saving throw. on a failed throw they take 7d8 of damage. on a  successful throw the target takes half the damage and returns to the battle. if they do not return to the battle on their next turn they make another saving throw. if they die in the pocket dimension they return to the battle as an undead creature under your complete control for 1 hour before being destroyed

every level you gain 1 extra d8