Source: Raiders of the Lost Ark
Location: Some warehouse currently
Threat Assessment: 10. If you could control it, I have no doubt you could destroy worlds. Luckily, for us non nazis, what's in the Ark has a mind of its own. Capable of killing nazis without much efforts and melting faces with ease.
Limitations: It doesn't seem to want to be active, thankfully.
Personal Note: According to lore the Ark is a tool of direct communication with God. Others say its a device by extraterrestrials. I leave it to you dear readers to sort it all out. I'm following Indy's advice and am not looking.