Writhing Vines

 are crafting materials that are dropped by the Swamp Demon and are used to craft Swampstone Platemail.

In order to get the 50 materials needed to craft the armor you need to defeat the boss up to tier #15. In order to gain the additional 50 materials needed to craft the plus version of the armor you need to defeat the boss up to tier #43.

This can also be obtained by summoning the Swamp Demon at summoning stone for 11 gems or afterwards 30 gems