Ancient Slime

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Ancient Slime is a boss from the SNES game Lufia II - Rise of the Sinistrals which is terminating one of the certainly most original levels ever in a SNES RPG, the Ancient Cave, in which you have to walk around through a randomly generated dungeon looking for stairs leading a floor down, despite the fact there are none leading up. You can only get out of the cave using the item called "Providence", which is hidden in a treasure chest below the -20th floor. The Ancient Slime itself waits for you on floor -99, which is quite hard to reach. You have to beat it in three turns (both Lufia games have a turn-based battle system), else it will run away and you will be left on floor -99 with nothing left to do but using "Providence", thus leaving the game. Anyway, there is a quite useful glitch for anyone who does not feel able to eliminate 4000 Hit Points in 3 turns, and that is killing your own party. After you have beat him, you receive a key for a treasure room and the last Iris Treasure, which is called "Ancient Slime" itself.