"The Crown Prince remained ill for a long time. Divination revealed a curse from Emperor Sudō.[1] Prince Chōshi, head of the Bureau of Mausolea and others were sent to Awaji Province to offer apologies to his spirit."
A week later: [Enryaku 11.6.17 (July 10, 792)]
"Imperial edict: “In Enryaku 9 (790) it was ordered that a certain Imperial Prince (later known as Emperor Sudō) be assigned a tomb and guardian household in Awaji Province. Moreover neighboring district chiefs were also appointed to this duty. However due to a lack of reverence in guarding the tomb there was a curse. From now on a moat around the tomb shall be constructed so that there will be no further irreverent defilement.”
Thereafter it was common to send messengers to pray at his tomb. In 800 Kanmu formally awarded him the title of Emperor, although it had been recognized previously, as seen above. This was apparently the only example of a person who never actually reigned being named posthumously an emperor. I hadn't known of his apparent connection with Zenju. But Sawara was a case of a priest later being named Crown Prince, then emperor. Fascinating stuff.
[1] 祟道天皇Imperial Prince Sawara, a son of Emperor Kōnin. He was presumably implicated in the assassination of Fujiwara no Tanetsugu in 785, and apparently exiled to Awaji, although this is not recorded in Shoku Nihongi.
Ross Bender