Swamp key: Opening a sunken crypt consumes the key. After The Elder you can buy more keys from the trader — and craft them too where recipes allow.
Boss rewards & summons: After you’ve unlocked the matching boss progress, you can buy or craft related rewards and summoning items — e.g. hard antler, wishbone, dragon tear, swamp key, plus things like deer trophies, ancient seeds, withered bones, dragon eggs, or goblin totems once unlocked. Check the trader and crafting menus for what’s available to you.
Boss trophies: Much heavier than vanilla and blocked from wooden portals. Plan transport (cart, ship, paths). Stone portals still work.
Buildable boss stones: Hammer (category “Boss Stones”), stonecutter in range. Unlock like other recipes — discover the materials first. Each stone needs, among other things, two trophies of that boss plus stone/flint/surtling cores and biome-typical materials. See the build menu for exact costs.