King Kill
A secret boss from my game Noel and the Tower of Doom. He is the self-described "king of bats" and is a creature who dwells in nightmares. He has the power to induce nightmares and fits of paranoia and possesses strong control over dark magicks.
To fight him in Noel and the Tower of Doom you must first obtain the Storehouse Key at the beginning of the Wind Ascent of Fornax-3. Then you must enter the storehouse and open the chest containing the cursed ring. This will cause the "nightmare bosses" to be freed and become available to fight. King Kill specifically is found on Floor 19 within a portal that looks like a crack in reality. Going through the portal will take you to "TRUE HELL" where all you do is walk forward before Kill King ambushes you and starts the fight.
The battle with King Kill is actually pretty simple. He mainly spams "Nightmare Gas" which inflicts sleep and poison on all foes. Then he uses "dream eater" to deal massive damage to sleeping foes as well as using "Bat Tornado", a powerful multi-hit dark spell, on awake foes. The trick to this boss is to simply have immunity to status effects, mainly sleep, which can be done with either attaching the rare Ankh Scroll to an equipped armor, the Cleric's "restore" spell (temporary), or by having certain charms equipped. Add onto that sleep immunity with resistance to or nulling dark and you're pretty much set!
King Kill's battle theme is the classical music piece "Night on Bald Mountain" and is unique to only him. He is inspired by numerous things including a dream I had when I was little about the "bat king" that made me terrified of bats for years, Nightmare from Kirby, and Anima/Night's Nexus/Vide from the Final Fantasy/Brave Universe cosmology.