History of Penumbra

Penumbra is one of two Taken Essence buffs used in the Vault encounter of Last Wish, the other being Antumbra. An Eye of Riven spawns in each phase and drops Taken Essence when killed; the Runner who picks it up sees Penumbra or Antumbra in their buff list.

Each plate needs one of the two. Destinypedia describes a plate needing Penumbra when its matching symbol is on the left and Antumbra when it is on the right. Some guides describe the left and right mapping differently, so check the in-game buff text before a run.