They normally do not have to be (demi-)god level, e.g. could be powerful pit fiend or high power fey as well. They give power in return for certain services from time to time, can even be a forced relationship. BG3 has a great warlock - patron example of this.
I could go with a generic powerful fey (wouldn´t even have to be an eldest), but I want to have the connection to the plot (which also gives me a good opportunity to foreshadow and steer things a bit in this otherwise quite open sandbox so that the main villain just doesn´t appear out of the blue after years of playing).
Ranalc could be quite hands off, give a few paltry abilities compared to his power level, not get involved in details but give the general task to find and recover Briar with the occasional nudge towards that goal. With Briar recovered, his former mate could recover her love for him and then work towards uniting with him (where ever he is nowadays...)
The other idea I had is Evindra (the nereid imprisoned in Whiterose Abbey who used to guard Briar) trying to find someone to free her.
But she would not be in a position to know much about what´s going on generally since she was always contained to her cave while Ranalc is just powerful and mysterious enough to drop in a few deus ex machina hints from time to time if required.