A DD cemetery key is a new item added in Elona+. It's function is tied to the Coffin of necromancy system. It is capable of storing 15 coffins into a single "space" shared by all keys. They can be randomly generated in dungeons, and one is statically generated as part of the Lumiest Graveyard. It weighs 3.0s.
While held in your inventory, the DD cemetery key will automatically return all deployed coffin of necromancy undead to their coffin state upon leaving a map. Note that this only happens with undead that are still "alive"; undead that have been defeated in combat and returned to their coffin state will not be picked up by this item, and so will be lost when leaving a map unless manually gathered.
So long as you have available inventory space, the coffins gathered by this item are placed inside your normal inventory. If any coffins are incapable of fitting inside your inventory, they are placed in this item's internal storage instead. When entering a new map, any undead that were turned into coffins by this item and are not in the item's internal storage will be automatically redeployed, given enough space exists on the map.
Any coffins stored in this item's internal storage - manually or as a part of its normal use - do not affect its weight. Coffins that stack are considered a single item so far as the internal storage limit is concerned.
Added in Elona+ 1.42.