Bat necklace was removed from RuneScape after an update, but still exists in-game for those who had obtained it.
The bat necklace is a reward from Treasure Hunter during the 2015 Hallowe'en event. It is obtained by redeeming 750 candy corn. It is purely cosmetic, and if stored in a keepsake the bat effect still shows. The necklace, however, will not vanish/be consumed, when stored in the Keepsake Box.