He probably treasures the most the times he spent with her, so she gonna take his memories of these times but i think it should have good ending so im curious what gonna happen.
@FgoSegul That would likely be the "intended price", but she could come up with a bullshit excuse like what is the memories of 1 year compared to a life time. She is basically Ctuhulu after all so might be immortal and therefore decide that staying a human lifetime with him may not be a bad way to get a little pause in the boredom that is the infinite void.
If wego with the idea that this is the prequel to the other stuff(which seems to be ao far) ultimately some sad stuff might happen here but they end up together.... very together.