It's not that complicated within the scope of things I can already do.

I offer my magic at the shrine, and I strengthen my bond with this Old God in particular. In return, I can use her magic when I need it. I guess it'd be considered "dark magic", which is why I can channel her version of necromancy. And before you ask: I don't hear her voice in my head, and I don't get weird nightmares. I haven't really felt any worrisome side effects.