Boss Stem Cells

Boss Stem Cells (or simply Boss Cells) are permanent items that increase the difficulty of the game when active. The first one is acquired by beating the Hand of the King with no Boss Stem Cells active, the second by beating him with one active, and so on. However, the fifth Boss Stem Cell is obtained by beating the Giant with four active.

Boss Stem Cells can be injected and removed at the start of the game in the Tube. Each one makes the game significantly harder by increasing enemy tier, density, variety, and limiting healing options. On the other hand, higher difficulty levels also increase cells dropped from enemies and the level of the Legendary Forge, and allow access to new blueprints and bonus doors scattered through the levels, as well as some ones dropped by enemies exclusive to high BSCs. The doors show the number of Boss Stem Cells required to enter and shine blue if enough are currently active. The fifth Boss Cell is special in that it doesn't modify anything compared to the fourth until the Throne Room, where it grants access to a secret biome, the Astrolab, and the hidden final boss of the game in the Observatory.

Trivia

 * When Boss Stem Cells, then named Boss Runes, were added in the v0.5.3 patch of the Foundry Update, they could all be obtained on 0 BSC difficulty and each was dropped by a different boss.