The Swallow's Cowrie Shell (燕の産んだ子安貝, "Tsubame no unda koyasu kai") is a necklace made with a swallow's cowrie shell: a special shell that is said to be a charm used by a swallow when giving birth. For some reason, Kohaku possessed this object, probably given to him by Naraku in order to lure out Kaguya. Naraku then compelled Kohaku to follow Kaguya back to her lair so that Naraku could attempt to absorb her into his body, though this ultimately failed. Therefore, the Swallow's Cowrie Shell was a piece that factored in both Kaguya and Naraku's plans. However, Naraku's plan was a bit of a gamble, as there was no guarantee that Kohaku would find Kaguya before she put a stop to time, though it is unknown if Naraku knew what Kaguya was capable of, and he probably underestimated her. At any rate, despite his orders to protect the shell from falling into Kaguya's hands, it was taken from following an ambush by Kagura. It signifies the element water and is represented on Kaguya's seal by the color blue.
Time has passed swiftly as I've waited to see the shell you promised: they say I wait in vain, could this be so?
Toshi o hete nami tachi yoranu sumi no e no, matsu kai nashi to kiku wa makoto ka
- The Swallow's Cowrie Shell is based on the one in The Tale of the Bamboo Cutter (竹取物語, "Taketori Monogatari").