Boss Stem Cells

Boss Stem Cells (also known as Boss Cells) are difficulty modifiers acquired by killing The Hand of the King. After you get the first, you must beat him with one Boss Stem Cell inoculated to get the second, with two cells to get the third and with three BSC active to obtain the fourth. The fifth BSC, on the other hand, is obtained by beating the Giant with 4 BSC active.

Boss Stem Cells can be injected/removed at the start of the game in the Tube, and make the game significantly harder by increasing enemy tier, density and variety, as well as by limiting healing options for the player. On the other hand, activating Boss Cells allows you to gather more cells from the enemies, to increase the level of the Legendary Forge as well as to get new blueprints and access bonus doors scattered in the levels. The doors show the number of cells required and shine blue if you can enter them. The fifth BSC is special in that it doesn't modify anything compared to the fourth until the Throne Room, but gives you access to a secret biome (Astrolab) and to the hidden last boss of the game (Observatory).