The Evil Force is a large, disembodied demon-like head with an aura glowing out of him. His main attack consists in lunging in to Shanoa, trying to touch her and deal some damage. If his prey is within range, he'll open his mouth and quickly stick out his tongue to attack with it. Evil Forces are found prominently in Mystery Manor and in Dracula's Castle, although they can appear in Minera Prison Island if a spotlight shines over Shanoa (this does not apply to the room near the end of the level, as the enemy is replaced by a Tin Man instead).
Fans of the series have noted Evil Force's uncanny resemblance to Gofer from the Gradius series, as Gofer was a demonic humanoid head created by the Dark Force (Bacterian). It is not uncommon for Konami to pay homage to their other games.
Evil Force may also be a reference to the main antagonist of the Evil Dead series, which is sometimes referred to as the "evil force". Additionally, it somewhat resembles the physical manifestation of the force seen at the end of Evil Dead II.