From Lexicanum

Kor Phaeron was Lorgar's spiritual advisor on Colchis and the Primarch trusted his counsel most of all. Kor Phaeron followed the Primarch through all his battles against the Covenant, providing him spiritual guidance as the battles seemed to have no end. As Lorgar took command of his Legion, Kor Phaeron became his second in command, leading the First Company of the Word Bearers.¹:

At the beginning of the Horus Heresy, Lorgar had received a reprimand from the Emperor for the long delays in making worlds compliant, and Lorgar retreated into his quarters for a month. As the chapter orbited planet 47-16, awaiting orders from their Primarch, Kor Phaeron came at odds with Sor Talgron, Captain of the 34th Company, over how the Word Bearers should handle worlds to be brought into compliance. Other than Erebus, the First Chaplain of the Word Bearers, Kor Phaeron is the only other to be permitted access to Lorgar during his month-long retreat. As Lorgar emerged from his seclusion, and commanded the elimination of the population on 47-16, it was Kor Phaeron's First Company that executed the remaining populace even after it became apparent that they were already worshipping the Emperor.²

Kor Phaeron became Master of the Faith, and it was his influence that eventually corrupted the Word Bearers legion, and led to the legion giving itself over to the Powers of Chaos.¹:

Sources

• ¹:Index Astartes IV - "Word Bearers", pp 14.
• ²: Tales of Heresy - A Horus Heresy Novel, pp 159-209.