Garmadon can pretty much do anything with the Megaweapon (within reason of course). Age manipulation, duplicating Ainz, time travel. Age manipulation works on things that were never young to begin with and interferes with their powers too. And touching the Megaweapon will erase you from existence. Also, Ainz will probably think the Megaweapon is a World Item and try and bring out his own to protect him, which would be a huge mistake here.
Also, this is Emperor Garmadon, who is covered in armor and may look like a monster, but his rib cage and other undead features aren't visible under armor.