The Ultimate Evil appears as a mass of mutated flesh comprised largely of heads, both human and canine. It hovers at either end of the screen, before flying off-screen and launching heads at you.
It takes about ten hits to kill it, possibly more. This battle is considered difficult as it usually hovers off-screen sending these heads at Rick.
Easter Eggs: Heads
It should be noted that the four heads it launches at Rick are that of other Splatterhouse enemies:
Blood Puppet from Splatterhouse 2
The Evil One later seen in Splatterhouse 3
One of the heads used by the Inverted Cross from Splatterhouse (1988)
Head D :
Zombie Dog from Splatterhouse (1988)
Connection to Splatterhouse 2010
Ultimate Evil makes an appearance as the heart of The Overlord during the final battle in Splatterhouse (2010), though it's never actually destroyed in the game, only thrown away. It is possible that this "creature" is the true leader of the Corrupted and was simply controlling the massive body of flesh much like how Balloon Head did in Splatterhouse 2.