Aeo was a capricious God above Gods. Aside from those deities that had existed since ancient times, many deities hadn't even heard any parables from this God above Gods. They were just unluckily turned into mortals with a slap when Aeo was angry.
Sometimes, when in a good mood, Aeo would fish out a Soul that had long since perished from the river of time and grant it the power of a deity.
Most of the time, this person's death was spectacular enough and interesting enough to talk about.
But Aeo almost never dealt with living mortals. No one knew why; it could only be speculated that it was this God above Gods' personal preference.
After all, the rules were set by Aeo, who held the absolute right to interpret them and to modify them at any time.
Amberser's death was legendary enough, and this lich himself had performed many feats beyond mortal limitations. It was quite normal for him to be deified by Aeo after his death.