From Destinypedia, the Destiny wiki
Splicer Keys are a consumable item, used to unlock loot caches in The Archon's Keep.
Splicer Keys can be obtained while playing Patrol The Plaguelands. Killing a Hive force led by a Brood Mother or a Brood Enforcer, guarantees the player a Splicer Key. The maximum amount of Splicer Keys a player can have at any time is limited to ten. Splicer Keys can then be used to unlock the passages into The Archon's Keep, which are normally cordoned off by laser grids. when you open them they lead to a room with a bunch of enemy's, an Iron Battle Axe to destroy them, and two chests.  Once in The Archon's Keep, the player will be able to find loot and relics out of chests, as well as the opportunity to start the Echoes of the Past Quest.
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