The description of Living Dead in the linked Google Doc doesn't match up with OP.

Living Dead: Holmgang equivalent skill. Unable to die for 15-20s, even if you reach 0 HP. You must be restored back to 100% HP before skill wears off, otherwise tank will die. Once 100%, buff will automatically fall off.
Living Dead - Applies a 20 second buff that, if hit fatally, allows you to survive, gives you ten seconds (of a “zombie” state) to be healed to full or you die (at the end of the 10 seconds). 300 Second cooldown.
You cannot die during the Pre-buff or the Zombie state.
As soon as you hit 100%, zombie is cleansed.
From the second description, the actual mitigation (zombie mode) only last 10 seconds.. same as Hallowed Ground. That 15-20 sec is reference to the initial buff, which on it's own, does nothing. It's the pre-buff: for 15-20 sec should you die you'll be rewarded with 10 seconds of extra life.

edit: knitpicking but sounds misleading