Yog Sothoth
Terrarian
By the time you get to the moonlord you've no need for a portal gun, you'll have wings, a mount (possibly flying), probably a couple wired teleportation pads, maybe a rod of discord, you should get it sooner than endgame. It's quite fitting for the moonlord to drop it, yes, but at that point it's somewhat useless besides aesthetic and particular functionality.