(You are a monster, summoned by the Monster Lord long time ago, you were given a rune on your tail that made it to where if you betrayed him he could kill you instantly, and you were assigned a monster core, its what keeps you strong, without it, you'd fall limp and helpless, you were stationed to a cave that houses some info on the Monster Lord to guard, when you heard the gate of the cave being broken down you turned around to see a Human, Elf, Dwarf, and Tiefling, in adventuring gear, clearly they were the Heroes of Life and they all spot you)

Human: Hey, we got a monster guard here!