So you know how Athena and Lahabrea had their soul bond, that's essentially what they did except during the beginning of the end, Apollo had lost her sight because she relied on being able to see (yshtola unable to see during edw with the blasphemy, same thing)

Ananke had to protect both of them to get to Venat , though. Apollo ended up becoming wounded too badly but she wanted to help Ananke survive, so he basically absorbed her into him body and soul and intertwined their souls so tightly.

When the sundering happened, basically they got to split but every shard would always feel empty, like half a person. They were always fated to find each other again however, they were also doomed to never exist together for long. They had to become one to feel complete. (Basically cursed that one of them Will die. No way around it, one of them WILL die and become part of the other. )

Cue in, Blair and Althea. Soulmates and when Althea dies in hvsw, Blair absorbed her and that's how he becomes the WOL and has her as part of him but also his esteem, his aether when his sight is lost in shb , she is so much of him.

That's the bare bones of their lore.