Black Crusaders

[Rasclomalum]

This is a heretical idea, but I just began thinking of a Black Templars crusade fallen to Chaos. The back story wouldn't be very difficult to draw up - something about the monastic vows, unquestioning faith and puritanical anger doesn't sit right with these guys (geneseed corruption? Sons of Dorn suddenly capable of thinking for themselves?), and because of crises/clashes with the Inquisition/whatnot the taint slowly grows, until one day they decide not to abhor the witch as much, and then it all goes to Jahannam in a spectacular display of unadulterated heresy, and likely gibbets of gore. Despite the Nucerian weapon chains, I feel Slaanesh is more fitting than Khorne here, in terms of devotion. Obsessive, compulsive rage and hatred for psykers is not the transgression here, but the freedom from repressive vows, the indulgence in all that life has to offer and an active interest in the psychic potential of the warp. They may feel abused by the Inquisition, Imperium and psychotic zeal of the rest of their chapter, and should have a good reason to fall to Chaos befitting a Greek tragedy.

 

A mix of Maltese Crosses and Slaaneshi/Chaos symbols could work just fine on them. Black armour, purple pauldrons instead of white? Their name is obviously a play on the fact that they used to be Black Templars, but participated in one of the Black Crusades (or just decided to launch a small one themselves). 

 

1) May have to be led by a Sorcerer instead of a Lord. It would be the ultimate transgression, though it doesn't unlock Noise Marines.

 

2) May have to make heavy use of Land Raiders, hurricane bolters magically having transformed to lascannons.

[Teetengee]

First I would say crusade rather than chapter, as Black Templars don't really have many successors I know of.
Also, I would make the Land raider with Warpflame Gargoyle Hurricane Bolters anyway, we might get them someday and it will look really cool.

[Rasclomalum]

We need more Land Raider variants! Better transports for sure. But that's a different topic. Now I need to write up a story. :-)

[Draakur]

When I read Templars turned bad, I can't help but think of the potential in the Dark Brotherhood, or at least the few visual details we know of them. Upon their corruption (for any of the reasons you mentioned), they would cast the white from their armour and lay a skull over their Maltese Cross iconography, symbolising the death of their prior convictions and oaths, and a new beginning...

 

The skull is red, so a Khorne dedication would be ideal :p But can't win 'em all! Just some thoughts :)