Sunny wanted to use the [Grace of Shadows] Ability of Soul Serpent to bestow it upon Revel.
There were two reasons why he had chosen to make that decision, each of equal importance.
The first one was pretty obvious, and had already been touched upon by Revel. He had to make arrangements for the Shadow Clan before leaving, the main of which was the fate of the Forgotten Shore.
Sunny and his forces were entrenched there, holding their own against the Barrow Wraiths. With Saint becoming Supreme, he was able to push the abominations back to the border of the Burning Forest — but if Sunny went away, the line of defense would collapse. In fact, if he did not take some kind of measures, the line of defense would simply cease to exist.
