User blog comment:ImperiexSeed/God vs Death/@comment-29551856-20160201082725/@comment-34326521-20160201090352

A few problems with your hypothesis:

Death could simply have never met Amara. He doesn't have to meet everyone and everything that has ever existed, no matter how powerful or ancient they are.

Lucifer lied throughout his speech in the cage. From saying he could defeat The Darkness on his own (in spite of his broken wings and the fact that, last time, it took 3 other archangels and God, and all they could do was imprison her) to saying Sam's visions were the word of God, he lied because he was desperate to get out. If Amara was as powerful as God, he wouldn't have needed 4 archangels just to imprison her. One would have sufficed. She's probably more powerful than he is, and by a considerable amount.

Death was concerned about Amara's release because she's powerful enough to end all life, thus rendering him powerless (another reason why he might have never met her: he didn't have the power he acquired later on, because there was no death). She's also probably the only true immortal in the series, so he'd be unable to kill her, even if he tried.

My guess, Death is equal in power to God, and Amara outranks them both.