You are undead. Most often, that means a necromancer raised you from the dead in a way that didn't entirely restore your life.

• You count as undead in addition to your normal type, as determined by your ancestry (usually humanoid).

• You have resistance to either poison or necrotic damage (your choice).

• When you roll a hit die to regain hit points during a short rest, you do not add your consititution bonus unless you are under the effects of the Gentle Repose spell.

If your character dies and is brought back from the dead in a way that would leave them undead, your DM may allow you to switch a feat for this one, or gain this feat as an extra feat.