Reliqua

 are consumables that one could buy in stacks of 20 for 1 from the Consumables Vendor.

When present in the inventory, these item allows the player to "Toggle Essence" (default hotkey: F). While toggled on, the icon will flash white, and each successful shot will deal double damage at the cost of one.

Tips
In the early version of the game where reliqua were purchasable (alpha build 3593), they were a very bad investment. Each successful shot would consume one reliqua, meaning that one stack would be consumed in seconds, even at the lowest levels. It was even worse at higher levels, where the fire rate would reach 10 bullets per second.