Fiends are entities summoned with a Special Skill which are applied to one or more targets.  Fiends can also be summoned by an event stage property. Fiends generally cause damage and negative effects to their targets, depending on the type of Fiend. The specific effects of a Fiend are shown on the owner’s Hero Card when active.

Healing targeted at a Hero with an active Fiend will be absorbed by the Fiend until its HP bar reaches 0. Once it reaches 0, it is removed. Certain Special Skills can also remove Fiends.

A single Hero can have up to 3 Minions or Fiends active at a time. If there are already 3 when a new Minion or Fiend is cast, the oldest one will be replaced. 

Read more about Minions here: What are Minions?