Dark Knight (Final Fantasy IV 2D)

Dark Knight is a boss from Final Fantasy IV. It attacks Cecil when he attempts to become a Paladin atop Mount Ordeals.

Strategy
The Dark Knight attacks solely using Darkness, which will do around 100 damage. However, using Darkness costs the user some of their HP, so Cecil merely needs to heal himself until the Dark Knight's HP runs out. The voice tells Cecil during the battle not to fight - although Cecil can defeat the Dark Knight with normal attacks, it is quite difficult.

Notably, in the original SNES release of the game, the Darkness command is not available to the player, so they would both be unaware of its origin and that it drains the user's HP.