Osiris was the Egyptian god of the underworld, and the one who authorizes resurrection spells.
Osiris has been shown to be a vital contributor to both Buffy's resurrection and the death of Tara. It is possible that he is seen for the first time when Tara's soul is being taken away from the human world. Willow brings forth an entity that comments upon her nerve to bring forth Osiris to beckon for Tara's life. It's unsure if this is Osiris for it refers to Osiris in third person, however this could just be Osiris making use of his title upon being called and is in fact the god of the underworld from Egyptian mythology.
Behind the Scenes
The Egyptian god Osiris was killed by his brother Set and sealed in a coffin. He was resurrected by his wife Isis for a short time so they could procreate. He was then resurrected as the god of the dead. The cult of Osiris was especially interested in mortality. These traits make Osiris a popular god in vampire fiction.