The Undead Groom is the posthumous version of the Conehead Groom, the boss who originally captured the Green Princess and attempted to marry her. Instead of being a standalone boss like his living counterpart, he is an invulnerable enemy occasionally released from the coffin carried by the Undead Cyclops.
The Undead Groom does not vary in attacks, regardless of the original. He follows the player around and charges from time to time, trying to knock the player down. This attack cannot be blocked, although the player can purposely get crushed by the Undead Cyclops to get stuck in the ground to avoid the attack. The Undead Groom is invulnerable, meaning that he does not get damaged in any way. He is only defeated once the Undead Cyclops is defeated, so it is meaningless to attack him. While he is charging, a discordant piano tune is heard.
The original Conehead Groom was defeated earlier in the game by the player in the Wedding Crash level. Later, after the Lava World level, the Conehead Groom and the Cyclops were shown to be resurrected by the Necromancer, the high-ranked, loyal servant of the Evil Wizard. The Undead Groom is seen with his visor lifted, revealing his cracked skull.