Overall, I think I have a pretty solid villain, but I have one problem.
He is supposed to be powerful, long-lived, and in control of lots of troops, yet he stays in the shadows until the MacGuffin arrives. The MacGuffin was lost somewhere on the island.

>Question:
How was he supposed to act?

Was he supposed to search for it himself? It's kind of like Freeza on Namek. (Probably not good for the campaign, but more realistic.)

Or would it have been better if he had just sat on his throne and waited for his troops to find it? It's similar to the first Homunculus from FMA or Ozai from Avatar. He has time, but, on the other hand, why wait if searching for it himself would probably be faster? Maybe I should think of a way to prevent him from moving.

Or maybe he was supposed to wander around from the start. Be a caretaker for the humanoids. He could treat them like cattle, unbeknownst to them. He wouldn't let cities and villages just die on the island. The PCs meet him early. He might have some quirks. He could tell stories from his childhood and talk about how he likes spicy food. This will make him seem more human and tragic when the final showdown arrives.