In Elden Ring, which boss is fought at the end of the optional underground area called the Lake of Rot?

At the end of Elden Ring’s Lake of Rot area, the boss is Astel, Naturalborn of the Void.

The Lake of Rot is an underground region reached through the lower part of the Ainsel River route. Its surface inflicts Scarlet Rot, so players must manage the status effect while crossing platforms, activating switches, and reaching the Grand Cloister.

From the Grand Cloister, a coffin carries the player to the arena of Astel, Naturalborn of the Void. Defeating this boss opens a route connected to Ranni’s quest and the Moonlight Altar. The encounter is optional for the main ending path but important for that questline.

Astel, Stars of Darkness is a separate boss with a similar appearance and name, found in the Yelough Anix Tunnel. The two Astel fights are therefore easy to mix up, but only Naturalborn of the Void is associated with the Lake of Rot journey.