Headcanon about Adar's Origin
I have a couple of headcanons.
He once served Feanor's household as a servant who loved Feanor and followed him across the sea and lost all hope when Feanor died, effectively "abandoning" everyone who loved him to the oath and Morgoth's wrath. I don't know why, but there's something about Adar being a victim of the oath and Feanor's madness and desire for revenge and Morgoth's machinations makes me go feral. I think I like it because while the Elves mourn (and/or condemn) Feanor through song and stories and they also mourn (and/or condemn) his sons, no one (that we know of anyway) really mourns those who followed Feanor. The unnamed faithful who committed the unforgivable act of kinslaying for their lords, who were also burned by the oath simply because they were loyal to their king/lords.
It's like in history when we talk about wars, we usually focus on the politicians and leaders, the "named" ones and it's considered niche history to dig into the one's who followed those soldiers and to trace the unnamed one's fate. Adar is the unnamed grunt, someone who sacrificed all for a lord and was destroyed by his loyalty, and has to somehow pick up the shattered pieces his "lords" left him with and build a life no one who hadn't gone through the same horror he went through can possibly understand.
In this headcanon, since he was one of Feanor's favorite servants and very close to him, I also see him trying to replicate that relationship with Maedhros, but Maedhros is not Feanor and he is not yet ready to embrace a servant so close to Feanor he was known as Feanor's shadow and so Adar is rejected by the sons of Feanor, a family he served and loved and wanted to be a part of (not in a romantic or sexual sense, but a communal sense, especially since he helped Feanor and Nerdanel care for the children the moment they were born). He still continued to serve them despite their rejections and he was with Maedhros when Maedhros was captured and that was how he fell into Morgoth's hands.
I also like to juxtapose Maedhros' rescue with the Elves who were not rescued, who were tortured and corrupted into Morgoth's servants. There's a painful tragedy that you can't save them all and even though Adar knows that logically, to be the one left behind, to know you were not prioritized for rescue...that fucking hurts. And that's when Sauron gets to him, promising him a lord who will honor his commitment, who will gift him a real family, who will give him children who will love him and remain loyal to him. Who will never turn him away...
The other headcanon I've been playing with is that Adar was a healer/midwife in Valinor. I haven't fully fleshed out this idea yet, but I think he's older then Feanor, but still takes Feanor as his lord, except this time instead of being part of the household staff, he works in the houses of healing and cares for pregnant Elves and their babies. He actually followed Feanor not as a warrior, like in my other headdcanon, but as a healer who hopes to open a healing house of his own in Middle-Earth. But of course he has to heal the fallen and then somehow ends up in Morgoth's hands and becomes the Adar we know and love.
I've also been playing with different headcanons regarding Adar's gender. He's definitely trans but probably genderqueer or agender, but I've also been playing with the idea that he was one of he first intersex Elves and that's why he went into healing because Elves just didn't know about any of the medical needs for the intersex community since Adar was one of the first...
