[Oiad]

Alpha Legion - Alpharius and Horus had continually been comrads despite their rocky introduction to each other in which the uninitiated Alpharius and his brethren had attacked the Sons of Horus. Before a duel between the two would meet any conclusion they came to recognize one and other for who they were and made amends. However the Emporer didn't give much time to Alpharius and the other primarchs, particularly Guilliman didn't have any respect for him (nice to see Guilliman being the reliable **** he is as normal)

Anyway, it was basically out of this lack of respect for Alpharius as an underrated tactician by everyone but Horus, which ultimately led to his, and thus the Alpha Legions switch of Alliegience.

I think that's right. And I also believe SteveSane is correct two about his analysis. The Priest was actually a chaplain of the Word Bearers, called Erebus who already secretly praised the ruinous powers. Though it wasn't a potion, Horus had been stabbed by a poisonous weapon wielded by the leader of a traitorous cult who praised Nurgle.

 

[Oiad]

I like your version best techmarine Rannon.

If you're trying to find basic fluff about it a good place to start is here. I find it quicker for finding out widespread stuff rather than searching through all the books again. And for those who think anything's wrong then source it and change it yourselves.