An Ancient Greek Necromancy Ritual is a branch of very dark art of necromancy originated from Ancient Greek culture. The ritual caused the dead to be reanimated and brought back to life as a "real zombie." It also granted superhuman characteristics to the reanimated person.
It was used by Neil to bring Angela Mason back to life. Ironically, he didn't know the effect of the necromancy ritual and his reanimation attempt caused her to become a vengeful zombie. Neil later died in Angela's hands.
Through the detail of ritual was not shown, the only known part of the ritual was carving a special sigil (which constituted by a triangle and some Ancient Greek letters) inside the coffin of the person who will be reanimated.
The reanimated person must forcefully put into their coffin and must be stabbed by silver stake in their chest, so they can rest in peace. But, it wasn't easy. Before this, the reanimated dead must be lured to the cemetery where they were buried by a "ritual" (ironically, the "ritual" was a fake ritual) which needed many things. Dean mentioned the ingredients which needed for the ritual was some candles, some black roots, some scar weeds, and many other many, unmentioned things. Ironically, Angela is lured easily with the fake ritual.