Any character or creature with the Enthralled condition automatically receives this feat.
When any source of damage would reduce a thrall's HP to 0, the thrall makes a DC 10 Constitution saving throw. On failure, the thrall dies with no other effect. On success, on the start of the thrall's next turn, the thrall regains 1/2 of its maximum HP, rounded up, as it rises from death in a zombie-like state. The re-animated thrall is now considered undead and is prone.
Re-animated thralls immediately die if released from enthrallment or after 2d4 hours. A re-animated thrall cannot re-animate a second time.
For CR calculation, any creature with this feat should increase its effective HP by 1/2 of its maximum HP.
See Enthralled for more details.