Drop through a trapdoor into The Binding of Isaac's Basement, side-on in Terraria: clear rooms, beat Monstro, keep the items.
You need both games: Terraria 1.4.5+ and The Binding of Isaac: Rebirth (Steam, any DLC). Isaac's monsters, items, sounds and rooms art are read from your own copy of Isaac; nothing from either game is in this download. On your first Play the mashup unpacks Isaac's art with Isaac's own official ResourceExtractor (about a minute, once).
A trapdoor appears next to your Terraria spawn point. Stand on it and press Down to drop into a fresh Isaac Basement floor, played side-on with your own Terraria character and gear.
In the Basement
Items you keep 13 Isaac collectibles on pedestals (treasure room and after Monstro) change your Terraria character for good: The Sad Onion, The Inner Eye (triple shot), Spoon Bender (homing), Cricket's Head, Pentagram, Polyphemus, Transcendence (slow fall and double jump), Charm of the Vampire, Tooth Picks, Lucky Foot, A Snack, Magic Mushroom and The Halo. Hover an item in your HUD to see exactly what it does in Terraria; right-click it to drop it for good, like a buff.
Beat Monstro, grab his item and climb out through the trapdoor. Your world is never changed: the Basement borrows a patch of the world while you are in it and gives it back exactly, and world saves always write the untouched world.
Single player. Multiplayer worlds don't get the trapdoor.
Credits
