Uumuu

Uumuu is a minor antagonist and boss in Hollow Knight. It is a large jellyfish-like creature that dominates the Teacher's Archive in Fog Canyon after Monomon the Teacher went into a permanent rest to seal off the Infection as one of the Dreamers.

History
While little is known about Uumuu's origin, it was created along with the rest of the jellyfish of Fog Canyon as the guardians of Monomon, one of the three Dreamers of Hollownest. When Monomon rested in the Dream World to halt The Radiance's efforts to flee, some of the Infection managed to escape, infecting multiple creatures of the fallen kingdom, the jellyfish being no exception. Uumuu, however, continued to guard the Teacher's Archive, attacking everything it considers a threat.

The Knight is sent on a quest to destroy the three Dreamers to open the seal to the Black Egg and become the next possessor of the Infection, and is accompanied by Quirrel at the Teacher's Archive, due to him being Monomon's assistant with amnesia. In the inner chamber of the archive, they are attack by Uumuu, who has no memory of Quirrel, forcing the duo to take down the beast to reach Monomon.

In Battle
Uumuu uses two main attacks to attack the Knight, both of which it uses electricity to charge an attack. The first of which it creates small bursts of lightning all over the arena. They are all randomized, but do leave enough room to maneuver. The second attack is where it creates bursts directly at where the Knight is standing. The burst trail continuously follows the Knight's movement, so constant movement is necessary to avoid getting hit.

Under its membrane, Uumuu cannot be damaged, as the Nail will simply bounce off the membrane when hit. Only Quirrel can pop the membrane, leaving the core exposed for a few seconds before the membrane reflates itself.

Uumuu has a short healthbar and only two attacks, but its attacks are fast and tricky to dodge and there isn't much room to safely heal lost health. Due to its erratic movements and sparse opportunities to hit it, the fight with Uumuu can become very drawn-out.

Trivia

 * The battle against Uumuu is the only boss fight in which an NPC assists the Knight take down the boss. The other two occurrences only occur after requirements are met: Cloth helps fight Traitor Lord after saving her in the Ancient Basin, and Hornet helps defeat The Hollow Knight after the Void Heart is obtained.
 * Uumuu is the only boss whose weakspot is temporarily protected.