Word of Death is a hex that once cast on a target deals very large amounts of damage (~10000 base) to the target when it expires in 20 turns. The hex can be removed by killing the hexer or using a scroll/spell of holy veil/rain. The spell is a magic special action and can be resisted with magic resist. It should be noted that if it is cast on a monster capable of splitting, any descendants of that monster will also carry the hex on them along with the remaining time.
Word of Death is a very powerful magic special action and if the player captures the pet that uses it, the pet can use it against enemies. Word of Death is powerful enough to deal large amounts of damage(50%+) to even Chaos Fort bosses.
The pet that is most likely to use this spell is the Messenger of death. Though this skill is not useful against the majority of enemies, it is unreasonably useful against strong enemies and bosses as even Chaos Fort and level 200+ dungeon bosses can easily be killed this way.
However, recent patches seem to have raised the general hex resistance of all monsters, and word of death fails more often than it used to. Still, a very powerful hex.