Penumbra

Penumbra is one of two Taken Essence buffs used in the Vault encounter of Last Wish, the other being Antumbra. Only one of three side rooms (Stairs, Tree, or Rocks) is open at a time, and an Eye of Riven spawns there. Its dropped Taken Essence grants the Runner the Penumbra or Antumbra buff, which appears in the Runner's buff list. The Runner then exits through the tunnel to the correct plate.

Each plate requires one of the two buffs. A plate requires Penumbra when its matching symbol is on the left of the player's set of three symbols, and Antumbra when it is on the right. Once two plates are known, the third is the opposite of the other two.