The premise of this is my character has a rival cleric of Lathander (he is a non-important NPC) that's hampering my characters machinations to spread the glory of Velsharoon! To deal with with him I am going to be using the brainkill spell from second edition to burn Lathander from his memories.

So what would happen to a cleric that could not remember their God?

PS. I'm trying to play a good clerk/wizard of Velsharoon, so if you can think of any other creative ways to deal this, let me know.

The spell says it removes the memory of a person, place or time period. A god is none of these things. I'd rule the spell fails to work.

Obviously, its not a place or time period. I'd also say a god is something far beyond a person. It would be like trying to remove the memory of a concept (which the spell does not allow); in many ways gods in D&D are, basically, the embodiment of a concept. Of course, your DM may say otherwise.

And even if it did work, the spell says things you've learned stay. It gives the example of forgetting the swimming lesson, but still remembering how to swim. So I'd rule the cleric kept all of its abilities, though they might not know why they can cast spells, they'd know that they could and how.

And, of course, just because the cleric forgets his god, doesn't mean the god forgets him. It would be well within the god's power to restore the lost memory.

And, Velsharoon was a lich and god of the undead. I'd think you'd just kill the dude.