"A great clammering of bones rises from the dungeon!"
The Skull Chain Necklace is a bosssummoning item. Upon usage it will summon 10 Skeletrons per Skull Chain Necklace in the stack if used during the night, and 10 Dungeon Guardians if used during the day, after which the entire stack is then consumed.
If 10 Skull Chain Necklaces were used in the stack, completing the swarm during the night rewards the player with a Boney Energizer. Completing it during the day rewards the player with an Extra Boney Energizer.