A greatsword given to a player in my dnd campaign, that was almost left behind. Designed for a fighter who wanted more gear progression than ability progression.

The Stoneblade is a +1 greatsword, that has a physical reaction to other magic items and creatures with magical resistance, glowing blue and shattering defenses like glass. With a ritual over a long rest, another magic weapon can be brought to the Stoneblade, and merged into it, copying [not adding] it's +X To Hit/Damage advantages and any other magical effects the weapon has. The merged weapon can be removed using the same ritual to remove the advantages from the sword, potentially replacing it with something else. Only one other magic weapon can merge with the sword at a time.
-Ultimately designed as a flagging system for magic resistances
-The sword will 'grow as you do', by feeding it stronger magic weapons
-Helps to find use for magic weapons that the party doesn't immediately have a use for