The Mush Mask Item Chain is a series of item trades that in the end, lets you catch Tumur, a Dark Legendary.
MUSH MASK: You find the Mush Mask in the Rustboro PokéCenter. You must take it to the scientist studying mushrooms on Route 123. He’ll trade you cologne for the mask.
COLOGNE: Take the cologne to Dewford, and enter the house where you found out your brother went to the desert. Give it to the thirsty guy, and he’ll give you a Spark Token. He’ll drink the cologne, then rush off to the hospital.
SPARK TOKEN: Take this token to Fortree, to Count D’s Pet Shop. The lady behind the counter will give you a Thunder Cub in exchange for the token.
THUNDER CUB: Go to Fort Draco and find the Endless Plains cave. Talk to the dragon there, and let your cub go. Come back later and you’ll find out he ate a sparkly pony, and he grew up into a Shinx. Remove the Cat’s Eye from his possession.
CAT’S EYE: Go to the sewers and find the girl who loves Shinx. Give her the cat’s eye.
That’s all I remember up to this point, I’ll update this page once I find out what the Shinx lover gives you.