[@DepressedSoviet] [@Jbcool]

[i]"Though it has happened only rarely, lone Space Marines in dark grey livery and speaking ancient variants of High Gothic have, on occasions, arrived at the fortresses of the Deathwatch in the Jericho Reach (and perhaps elsewhere) in order to "fulfil the obligations of their sworn oath." When pressed, some of these Space Marines have said they hailed from a Chapter known as the Carcharodons. However, beyond these rare encounters recorded in the dusty data-vaults of Watch Fortress Erioch, the only information forthcoming on the Carcharodons are the stories of their fell deeds throughout the Imperium."[/i]

From this page: http://warhammer40k.wikia.com/wiki/Carcharodons 

Good enough for an RP, I reckon.