Essence Drainers are FOEs that are in the secret area of B15F. They will not appear unless you are in a battle. During a battle, they will become agressive and can move multiple squares per turn in a straight line towards the player. They will disappear as soon the player is out of battle. Between 1 and 3 Essence Drainers can spawn at once depending on the room. The easiest place to battle an Essence Drainer is the room in the South Eastern side of square B3, since only 1 Essence Drainer will spawn in this room.