I know that Phra Narai (Narayana) declared that he will reincarnate as a human (Phra Ram/Rama) and kill Thotsakan but when Thotsakan was reborn as the demon king of Lanka and started conquering the three realms, wouldn’t Phra In and Phra Isuan have both stepped in and said “hehe sorry buddy you’re not doing this”? Why didn’t they directly intervene against Thotsakan and put him in his place?
Reference : https://en.wikipedia.org/wiki/Jaya-Vijaya
In Hindu mythology, Jaya and Vijaya are the two gatekeepers of the abode of Vishnu(Narayana OR Phra Narai), known as Vaikuntha(place of eternal bliss which is not included in three realm). Due to a curse, they were forced to undergo multiple births as mortals who would be subsequently killed by various avatars of Vishnu(Because they were his gatekeepers he decided to kill them by taking birth with them). They were incarnated as Hiranyaksha and Hiranyakashipu in Sat Yug, Ravana and Kumbhakarna in Treta Yug, and finally Shishupala and Dantavakra in Dvapara Yug.
These births include RAVANA , so Narayana has to kill him as a Mortal.