A Vow in the Dark

Both are xianxia dramas in which the MLs are demon lords, and the FLs immortals who have a past life connection to the ML. Both SMLs are the FL's master who she is devoted to but has secrets of his own. Both plots have the FL thinking the ML betrayed her and later discovering the higher immortals are manipulating her, pushing her into killing the ML as she did in the past.

Both about a Demon Lord and a girl who has special connection and history to the demon lord. both Demon Lords are nice to the FL, and protect her.

It looks like a shortened version of Love Between Fairy and Devil, that is, some kind of knock off. The plot is so similar and the first association during and after watching it similarity between this two dramas