Turn a Prismatic Shard into a Galaxy Sword
Take one shard to Calico Desert and keep it selected before stepping between the pillars. You do not need the Skull Key to perform the outdoor exchange.
Take the correct shard to the desert
Check the item name in your inventory: it must say Prismatic Shard. A rainbow-coloured mineral of another type is not a substitute. If this is your first shard, decide whether you prefer a museum donation or the immediate weapon; the pillar exchange consumes one. Travel by repaired bus or a desert warp and head for the three standing pillars in the northeastern area, not the cavern mouth in the northwest.
Cross the middle tile while visibly holding the shard
Select the shard on your hotbar so the character raises it. Walk to the center of the triangle rather than clicking the pillars from outside. The exchange animation replaces one shard with the sword. If no animation starts, move away, reselect the shard and walk back to the middle. On a modded save, check the mod’s unlock conditions: players have reported overhaul mods changing this exact puzzle.
Replace or change the weapon at the Guild
After the first unlock, Marlon sells additional Galaxy weapons at the Adventurer’s Guild. A replacement Galaxy Sword costs 50,000g in the base game; the Galaxy Hammer costs 75,000g. Check the Guild shop if you lost the weapon rather than sacrificing a second shard to the same puzzle. Shopping requires access to the Guild, so finish its Initiation quest if the door is still closed.