Oblivion

Oblivion is a higher being she is one of the three river stixx. She appears in the book Bobby Singer's Guide to Hunting before the apocalypse had been stopped she had been a servant of Heaven her job was to erase the memories of someone who had seen something they weren't suppose to see.

What she wanted was a memory of Bobby's that was his chief reason for living through everything he'd been through. It's never stated what his memory was though Dean speculates it could be about his unborn child. Oblivion had no way of finding Bobby so instead used a Banshee to lure him to her.

After escaping his memories were still susceptable to her so she tried to lure him back by feeding his memories as he wrote his guide. It was only by feeding her their worst memories that they were able to over come and kill her. After which Bobby smiles and says "It ain't dead till it's in a million pieces." A reference to his friend Rufus Turner