The prestige class black blood cultist (Champions of Ruin 44-8) gains at level 6 as part of its extraordinary ability feral rage this ability:
At 6th level and higher, whenever you hit with both of your claw attacks during a rage, you rend your opponent’s flesh, automatically dealing double claw damage in addition to normal damage. (45)
Thus, after the raging black blood cultist makes a second successful claw attack, the black blood cultist automatically deals his foe an extra 2d6 points of damage, and more than that if he's bigger than Medium. (Tip: He should be!)
To be clear, this damage is in addition to any damage he's already dealt with the two claw attacks and doesn't require an additional attack roll. However, this rend damage goes unmodified by additional effects beyond the cultist's claw damage (so, for example, the feat Power Attack wouldn't increase this rend damage, but an enhancement bonus that directly increased the cultist's claw damage, like that of the spell magic fang, would increase this rend damage).
Rend damage is fixed and disassociated from the character's standard damage. When it reads claw damage in the description of the black blood cultist's rend attack, it means only the claw damage, not claw damage modified by Strength. The Rules Compendium clarifies this in its section on Rend:
If a creature that has this extraordinary special attack hits with the specified natural attack, it latches onto the opponent’s body and tears the flesh. The rend attack deals damage equal to that dealt by the creature’s natural weapon + 1-1/2 times its Strength bonus or its full Strength penalty.
Because the cultist's rend is a specific kind of rend, not the general kind mentioned by the Rules Compendium, the cultist's rend follows the RC's rules until it doesn't, which leaves the rend damage unadjusted by the cultist's Strength score.
I assume the DM will mandate that both attacks must be made against the same foe, although the text only implies this. The DM must also rule how this special ability interacts with the feats Rapidstrike (Draconomicon 73) et al. (assuming the black blood cultist's type is an aberration, dragon, elemental, magical beast, or plant and he takes such a feat, of course).
(Also, don't confuse rend with rake; they're totally different.)