The Evil Force is a large, disembodied demon-like head with an aura glowing out of it. Its main attack consists in lunging toward the player to inflict contact damage. If close enough, it will open its mouth and stick out its tongue to attack with it.
This enemy is prominently found in the Mystery Manor and in Dracula's Castle, although it can also appear early in the game if the player is discovered by a spotlight in Minera Prison Island (this does not apply to the room near the end of the level, where a Tin Man appears instead).
It also resembles Gofer, a recurrent antagonist from the Gradius series. Gofer was a demonic humanoid head created by the Dark Force (Bacterion). It's not uncommon for Konami to pay homage to their other games.
The Evil Force may also be a reference to the Kandarian Demon, the main antagonist of The Evil Dead series, who is sometimes referred to as the "Evil Force". In addition, it somehow resembles its physical manifestation, being a fleshy disembodied head with large teeth, as seen at the end of Evil Dead II.