Faustus Blackwood is the instigator of many calamitous events in Greendale and the Church of Night. After the Spellmans use his body to trap Lucifer, he and Lucifer team up to escape. Not only does he get the Mark of Cain from Lilith, which means he can't be harmed, he joins forces with the pagans to get back at the Spellmans. Right before the pagan apocalypse, he wreaks havoc at the Spellman house by killing Zelda and Prudence.
Even after Sabrina reverts the world back to "normal," by the end of the season, Blackwood takes Agatha and his twins into the forest with the magical egg and cracks it open, unleashing the "Eldritch Terrors" into the world.