Death Becomes You

You did not die when you should have, and because of that Death changed you. 

You count as an undead creature for the purpose of spells and effects (such as Turn Undead).

When at or below 20% of your maximum health points, you emit an aura in a 10-foot radius. At the end of your turn, this aura deals 1d8 Necrotic damage to all creatures within the aura's radius, except you. You absorb as HP,  half the damage caused to any creature that survives, and the full damage caused to any creature who dies. Any creature killed by your aura begins immediately to decompose. The aura persists until you are above 10% of your maximum health points.

The radius and damage of the aura increase at higher levels. (15 ft and 1d8 at 6th; 15 ft and 2d8 at 10th; 20 ft and 2d8 at 14th; 20ft and 3d8 at 18th)