Wilson was asked how he approached creating prosthetics for Adar and communicated that there was something wrong with him. He explained that they worked to keep him looking like “some kind of beast or over-the-top.” The idea, he says, is that the audience realizes he’s an elf but sees that he’s turned into the dark. If we do finally see Sauron when he first appears before the elves in Tolkien’s work later in the series, it’ll likely be the opposite of this; Sauron’s deceitful form is beautiful and is referred to by Annatar as the “Lord of the Gifts.”
Wilson explains that keeping Adar’s goblin appearance “adds more story to it, rather than just some crazy monster:”
“So it’s important that we definitely keep the spirit there, but at the same time there’s darkness creeping in, possibly matter – because obviously, you can see he’s been attacked. or was traumatized at some point – and possibly mental darkness as well. that crept into his soul.”
There was something that made Adar feel “down,” aside from his actions. He seemed not only hurt, but damaged. His elf heritage is obvious, but there is a haunting look to him. It is an unusual and grayish appearance to the skin commonly seen in people with a particularly high fever.
https://www.slashfilm.com/1012987/how-the-rings-of-power-kept-an-elfness-to-the-look-of-adar-exclusive/ How the rings of power hold ‘fairy’ in the sight of Adar [Exclusive]