Kamaloka quest. Not needed to enter or complete either one of the instances but provides small adena reward for killing both bosses in the Hall of Abyss (each captain has 2 level ranges for those and then one for Labyrinth of Abyss).
All you need to do is take the quest, complete both Hall of Abyss instances from that Guard NPC and go talk to another NPC (usually in temple of the same town) to claim your adenas. Endlessly repeatable and can be registered unlimited time until you get the items from both mobs.