A fiend is a rat-like monster that appears in rooms you've already cleared. Fiends are oblivious to you for a bit unless you approach them. They can rapidly shoot out red projectiles that deal a small amount of damage that racks up quickly. Approximately 2.32 fiends spawn per cleared room. Fiends will never spawn in the Spiritual Dungeon.
In about 1/3 of the cases they drop a health potion. In fewer cases they drop a mana potion (below 1/30, maybe 1/50 till 1/100).