Ritual of Doom is a Warlock spell that when cast, sacrifices a participating party member's life for the benefit of summoning a Doomguard. It requires the caster and four other party members to participate in the summoning. It is granted to all warlocks at level 31.
This summons a hostile Doomguard that soon becomes automatically subjugated as an additional guardian pet for the Warlock. Currently, the Doomguard lasts up to 15 minutes, but has a small chance to break free from the subjugation at any time and attack your group. This is close to the way the old spell was designed. Originally, the demon had to be enslaved or otherwise it would attack the party. It is related to the Doomguard that spawned from the proc of Curse of Doom, which needed to be enslaved and would break free from enslavement after a certain amount of time had elapsed. Ritual of Doom was later updated to require only a significant portion of the caster's health to summon. Then, the spell was removed from the game entirely and replaced with [Summon Doomguard], which simply summoned the Doomguard for 5 minutes, after which it would despawn.
Patch 9.0.1(2020-10-13): Re-added. Now learned at level 31.