No one understood the barrier between life and death better than Tirian and his undead sailors.
The Death God Bartok had a door that anchored the boundary between life and death; in short, once the soul of a living person passed through that door, they would arrive in the world of the dead, and that door was one-way—in other words, unless that door had been passed through, neither a temporary reawakening of a corpse nor a persistent undead curse could truly be considered "resurrection from the dead."
"Many people in the world confuse 'undead' with 'the dead,' even thinking the former have exploited a loophole in the door of the death god," Tirian turned around, looking at the clamoring sailors in the square, and said calmly, "but in fact, they are only rejected by that door due to soul contamination, hence being stuck on the brink between life and death. According to the strict concept of the Death Church, 'undead' actually belong to the world of the living."