Guarding the Wizard Tower for 100 years, I Become Invincible When I Come out

Peter transmigrated to the wizard world and became a magic apprentice at the lowest level of the wizard tower, responsible for guarding the monsters suppressed by the wizard tower. He couldn't even do the most basic fireball magic, so he could only do some simple things. He would become the monster's food if he was not careful. The appearance of a magical book changed all this. Watch over the Mountain Giant for twenty years and gain the forbidden supernatural petrification spell! Watch over the Purgatory Fire Dragon for thirty years and gain the dragon language magic, Dragon's Spit! Watch over the Frost Giant for fifty years and obtain the forbidden magic of ice, Frozen for Miles! Watch over the Dark Phantom for eighty years and gain Super Charm... A hundred years later, the Abyssal Demon King descended from the abyss with the Abyssal army, and the Wizard Tower faced the disaster of destruction. A magician wearing a magic apprentice robe came out from the lowest level and looked at the Abyssal Demon King with just a gentle wave of his wand. Instantly, the abyssal army flew into dust!