Kasuga faith and faith in the historical Buddha Shaka (Skt. Śākyamuni) are bound together in this portable shrine. The original form (honji) of one of the Kasuga deities was believed to be Śākyamuni Buddha in the religious paradigm of honji suijaku. The relics of Śākyamuni—his bodily remains—are enshrined within a reliquary that rides upon the sacred deer, the mount of the deity, who appears here in the form of Śākyamuni’s relics.
 
NAITO SakaeEnglish by Mary Lewine 
Buddhist Art Paradise: Jewels of the Nara National Museum. Nara National Museum, 2021.7, p.335, no.173. 

This palace-shaped feretory is made of black lacquered wood and is known as "Kasuga-zushi" (feretory of Kasuga). This feretory has a rather flat roof and is placed on a two-storied basement, which is decorated with stripes and kōzama openwork decorations. A gilt-bronze holy deer is placed on a cloud-shaped wooden seat within the feretory, which represents the emergence of the Shintoist deity in Kasuga. A relic-container in the shape of a flaming magic jewel is set on the back of the deer. This arrangement is based on the Buddhist-Shintoist idea, which asserts that Kashima-myōjin of the first sanctuary in Kasuga shrine was an incarnated image of Shaka (Śākyamuni or Buddha). Therefore, the relic on the deer-back in this feretory represents Shaka.
Worship of Buddhist relics was popular during the Kamakura period in and around Nara, and many relic containers and feretories were made. This is one of them. It is an important object which shows the close relationship between the worship of Buddha (Buddhist relics) and the worship of Shintoist deities in Kasuga. 
Masterpieces of Nara National Museum. Nara National Museum, 1993, p.108, no.84.