I just got done reading the Omnibus, and i thought it would be interesting to play a Soul Drinker who is in the Deathwatch when his chapter falls unwittingly, much like how Astral Claws can be played right as the chapter as a whole falls to Chaos.

I think that the best answer to that is do you and your GM want them to have fallen yet? Even if the GM decides they have fallen, you could always play a Black Shield who was a Soul Drinker.

E

The only referance I can find to when the Soul Drinkers were excommunicated is 'the late 41st millenium'. So it's either recent past or near future for stock Deathwatch campaigns. Eldath already had the right answer , but with no canon date, there's no canon answer, unlike the events of the Badab War.