User blog comment:CaptainAndrath/Alternate Heresy/@comment-24459347-20140807132721

Lorgar sort of did start the Heresy when he felt betrayed by the Emperor, Erebus and Kor Phaeron were the actual starters of the Heresy, when Horus was wounded and being treated, Lorgar, Erebus and Kor Phaeron put visions in Horus with the help of the corrupted healers of Horus. So on conclusion: Lorgar.