Enemies-to-lovers works because the distance the characters have to cross is the whole story. The further apart they start, the better it lands. So what happens when you make them immortal, and put a seven-hundred-year-old feud between them?
The trope, briefly
Two people who should not want each other, who start on opposite sides, and who fall anyway — usually against their own better judgment. The pleasure is the slow collapse of the reasons they gave for staying apart.
Why immortals make it worse (in the best way)
A mortal enmity is a few years old. An immortal one can be centuries deep, tangled with family, obligation, and a peace that other people died to keep. When the two leads finally turn toward each other, they are not just risking a relationship — they are risking a treaty. That raises the cost of every good decision and makes every bad one understandable.
In the Bloodbound Houses
The lead and the House heir sent to deal with her begin on opposite sides of a line drawn in 1326. He has orders. She has just learned what she is. Neither is supposed to want the other, and the reasons they give for staying apart are real ones — which is exactly why watching those reasons fail is the point.
The further apart they start, the better it lands.