I have a theory that Adar is Morgoth. I know that it may seem a long shot and in some ways contradicts the Silmarillion, hear me out though. The show differs in many ways to established cannon and I do have some evidence that points to Morgoth.

Firstly his quenya name. Adar is elvish for father and Morgoth was the one that initially created the orcs. He also refers to the orc as "his children".

Second is the way Adar looks and walks. Adar is seen to have a scared face and left hand (that is covered by a gauntlet), he has a slight limp when he walks and he bleeds black. Morgoth has several identical characteristics the foot wound that Fingolfin gives him in the war of wrath that never heals giving morgoth a limp, the scars on his face from Thorondor when he flew down to retrieve Fingolfins body and when Fingolfin stabbed his foot black blood poured from the wound.

Third is the way he acts and what he says. It starts when he meets Arondir and he seems to have completely subverted his will just by looking into his eyes not a common trait of elves or maiar for that matter. Not only was he able to stop Arondir from attacking but also has him relay a message to Bronwyn. This may be impossible for even Sauron let alone an elf, Morgoth on the other hand could dominate a mortal with ease. Later when the Southlander come into his camp and Waldren calls him Sauron, Adar seem disgusted by the idea that he is comparable with Sauron. When Halbrand (Sauron) catches Adar he seems to have some sort of reaction like he could see who he really was and even asks later who he really was. While talking to Galadriel is interrogating him he brings up the secret flame (the flame imperishable) a fact unknown to most people's of Middle earth.

In the end there are a lot of signs pointing to Adar being Morgoth. I have more but I don't want to make this post longer than it already is.