Observatory

The Observatory is the lair of the The Collector, who is the hidden true last boss of the game. This level can only be reached when playing with 5 BSC active and reveals the true ending of the story. When the Beheaded reaches this biome, the Collector reveals his true identity as the Alchemist and asks him to deposit his cells in the Catalyst next to him. By using the Catalyst on the Cells, the Collector intends to brew the Panacea, a fabled cure to all diseases. Although the Collector doesn't believe the legends passed down by the first Alchemists of the island, he attempts this experiment in a last-ditch effort to cure the Malaise and save the island as well as himself. It turns out the remedy works, and thus the Collector tells the Beheaded to go back and gather more cells, which triggers the fight.

During the fight, which consists of a number of phases in different rooms, the Collector keeps healing by drinking the Panacea. Once the player has dealt enough damage to the boss and/or survived through all phases, the Collector teleports himself and the Beheaded in the last room. Then, he becomes vulnerable when he attempts to drink, and drops the Panacea when hit. The player can then pick up the Panacea, which fills his potion slots with a blue liquid. If the player drinks it, their attacks with main weapons become extremely powerful and quickly deplete the Collector's health, leading to his downfall. Finally, the Beheaded ends the Collector by kicking his face to the ground. It is at this moment that he realizes that the cure is effective, and that he doomed the island's fate by killing the only person able to recreate it. Despite this, the Beheaded doesn't really care, as he believes the Panacea has cured him from the Malaise and will allow him to finally leave the island... although his wishes are ultimately not granted.

If the player reaches the Hand of the King a second time with 5 BSC active, he can possess the King's glitched corpse by using the Homunculus Rune and thereby repossess his own body. Then, a special ending can be seen by beating the Collector again while wearing the King's outfit. This permanently unlocks the King's outfit.