What is the name of the hidden boss in Ulduar that requires a special quest?

The hidden boss in Ulduar that requires a special quest is Algalon the Observer.

Algalon is an enigmatic celestial watcher sent by the Titans to evaluate Azeroth after the world’s systems were disrupted. Unlike the raid’s ordinary bosses, he is an optional encounter hidden in the Celestial Planetarium. Players must complete a special attunement questline involving Ulduar’s Titan machinery and obtain the necessary access before they can face him.

The encounter was designed as a demanding test of raid coordination. Algalon uses powerful abilities such as Cosmic Smash, which creates dangerous impact zones, and Big Bang, an enormous periodic attack that forces players to use the encounter’s defensive mechanics correctly. His arrival also gives Ulduar a broader connection to the ancient Titan-created structure beneath Azeroth.

Freya and Mimiron are important Ulduar bosses, but they are part of the main raid progression. Yogg-Saron is the raid’s climactic Old God encounter. Algalon is distinct because he is secret, optional, quest-gated, and tied directly to the Titans’ judgment of Azeroth.