Ashes of a Phoenix.
Phoenix ash is remnants of a Phoenix. Sam and Dean use it to kill Eve. As Bobby puts it, "ashes of a Phoenix can burn the Mother". It's the only thing that can kill her, likely other than by God or Death.
Dean drinks a shot of alcohol containing the ashes, thus introducing the Phoenix ashes into his blood. He then goads Eve into biting him. Eve is poisoned by the ashes and dies.