The Ophidium Gulf Crusade was an incident in the recent history of the Black Templars Chapter of Space Marines in which contact was inadvertently made with the Fallen Angels, the hunted Traitors of the Dark Angels Chapter.
Not all the Crusades of the Black Templars are vast undertakings, as some may be comprised of as little as one hundred Initiates. In 998.M41, one such Crusade was launched by Castellan Raimer, the Master of the Black Templars' Strike Cruiser Ophidium Gulf and its company of 90 Black Templars, into an unexplored region of space in the galactic south called the Veiled Region by Imperial stellar cartographers.
There, the Black Templars found a small alien empire located in a single star system. The xenos lived in great cities of gold and followed the commands of a theological leader which they addressed as "the Voice of the Emperor." Curious at this strange similarity to the Imperial Cult, Castellan Raimer destroyed the temples before continuing into the system. As they came across more settled planets, the Black Templars found that some of them had already been destroyed by an unknown force even greater than themselves.
The Ophidium Gulf was a fast vessel and soon caught up with the interlopers, a small fleet of Space Marine starships which the sensorium of the Ophidium Gulf later confirmed belonged to the Dark Angels Chapter. Raimer promptly offered the fleet commander, an unusually tall Astartes (who never removed his helmet), the Black Templars' aid in their mission, aid which the Dark Angels only very reluctantly accepted.
As they neared the centre of the star system, on the most densely populated planet, the Space Marines together fought their way through a horde of crazed alien religious fanatics until the Black Templars captured the Voice of the Emperor and learned the source of the aliens' strange religion. The Voice of the Emperor was a single Space Marine in ancient Mark I Thunder-pattern Power Armour dating back to the Emperor's Unification Wars on Terra in the 30th Millennium which was black with no adornment or livery of any sort. The Black Templars took the unusual Astartes prisoner and locked him away in the deepest, most secure holding cell of the Ophidium Gulf. No sooner had he done this than Master Raimer was visited by the Dark Angels' commander (who still refused to show his face) who demanded that Raimer hand over the prisoner to his custody. It was only when the Dark Angels' fleet began to ready their weapons that Raimer relented since his single strike cruiser was no match for the Dark Angels fleet and he released the prisoner to the other Space Marines.
Before leaving, the Dark Angels' commander finally removed his helm, revealing a face twisted with scars and adorned with the symbols of Chaos. Before the Black Templars could react, the "Dark Angels" teleported back to their fleet and disappeared into the Warp without another word. This is one of the only reported instances of a Space Marine Chapter (other than the Dark Angels) coming into contact with the infamous Chaos renegades known as the Fallen Angels.
The ultimate fate of the Ophidium Gulf is unknown, as a single astropathic communication from the starship was received just before it prepared to make a Warp jump from the aliens' system and no further communication has been received by the Black Templars since then.
The fate of the ancient Space Marine who had been responsible for the creation of the alien species' religion remains unknown.
- Codex: Black Templars (4th Edition), pg. 20
- White Dwarf 312 (UK), "Righteous Victories"