This is the final boss of the game, able to use 6 different attacks, one for each of the 6 types in the game. The Fire attack consists of 24 balls released in an arc shape formation, constantly damaging characters. The Ice attack summons 10 sparks which fall down while producing mine residues, both of which slow the characters it strikes by 20%. The Thunder attack shoots 6 lasers towards random characters (including dead ones). The Poison attack shoots 20 spears up the sky which creates a cloud residue when hit, both of which will poison characters. The Freeze attack shoots a bullet at extreme velocity that reflect off terrain, freezing characters and creates a large snowflake which may freeze another character who happen to be close to the flake. The Physical attack creates a skull-shaped attack on a random character (including dead ones), instantly killing all characters staying within the projectile after 1.8s.
If the last attack just over ~1.2s ago was the 3.6s Freeze Bullet and the skull attack appears on the frozen character, there is no escape as the skull attack will hit at ~3s of the 3.6s freeze time. At least one (preferably high-LP) melee character with a Black Crystal 7 + Freeze Charm 4 equipped to attempt to draw most of the freeze bullets during the fight is highly recommended, but remember to watch for Skulls appearing on other characters as well. Beware of the possibility that the boss starts the battle with the skull attack; since the characters start off bunched together on the left of the screen, the skull will appear over all four of them and immediately cause a game over if they are not immediately moved out of the way.
Before ver18.5, the Hell Castle Boss does not have the Ice resistance, making this the second enemy being strengthened after its release.
Every stat on this enemy that can be seen in the book consist of the number 9 only. (LV 99, has 999999 LP, gives 99999 EXP, and drops 9999 gold)
As a result from this, this enemy has the most LP and gives the most EXP out of any enemy in the game.
Currently a bug exists where the extremely high EXP output may crash the game if the player defeats this boss with several Iron Medals 6/7 due to the EXP gained raises characters' LV by more than one.
This enemy's skull attack has the highest physical AT in the game, always doing 9999 damage. The previous record holder for highest AT, the Grey Big Box Snake, does random damage from 1-9999, and now holds the highest elemental AT in the game.
Despite this enemy's Skull head looking like a new version of the Skull heads, it is actually a normal Skull head with a crown image attached by a conditional of enemy ID in the game's source code, which can be considered as a new head.
This enemy has the second longest freeze time for an attack in the game, behind the Submarine Shrine Boss (5s).