Honestly, I've always found fics where Elijah's loyalty is weaponized against him by Klaus to be the most heart-wrenching. The endless 'always and forever' becomes a collar, not a vow. There's this brilliant one on AO3, 'Castle of Cards,' that had Elijah meticulously arranging peace between covens, securing alliances, only for Klaus to stroll in and tear it all down for a perceived slight. The story didn't have Elijah rage or leave; it ended with him cleaning up the mess, his internal monologue utterly hollow. That quiet acceptance hits harder than any tantrum. It shows his loyalty isn't blind love, it's a foundational, pathological part of his identity he can't excise, even when he knows it's toxic. He's not a martyr, he's an addict, and the family is his drug.
You also see it in fics exploring his dynamic with Finn or Kol. He's the older brother who failed them, a different kind of loyalty failure. Stories where he tries to make amends centuries later, bringing them books or securing artifacts they'd like, are a subtle, sad contrast to the loud, destructive loyalty he shows Klaus. It’s like he’s trying to be loyal to the idea of the family they could have been, not the monster it became.