Bleeding -X: If this attack deals damage to a creature, roll the d12. If the d12 is rolled 2X or higher, the attack deals bleeding damage. This means that during the next upkeep only, the damaged creature takes unavoidable, critical damage from X dice.
Example: I have a creature. This creature has an attack that reads "3 Bleeding -2." I attack a creature that has 0 armor and 5 life (for simplicity's sake) with this attack. I roll 3 attack dice. I roll 1 blank and 2 normals 1s, so I deal 1+1=2 damage to the target creature. Since I damaged the creature, it might take some bleeding damage, so I roll the d12. X=2 in this case, so I must roll 4 or higher to do bleeding damage. Let us say I roll a 6; bleeding damage takes effect. During the next upkeep, I will roll 2 dice and the damaged creature will take whatever damage I roll.
More powerful creatures could have insane things, like attacks like "4 Bleeding -5". Direct bleeding damage from 5 dice might sound cheap, but keep in mind: a) the attacker would have to deal damage AND roll 10, 11, or 12 on the d12 just to get the effect; b) rolling 5 dice is no guarantee of anything extraordinary; c) you could give creatures with high bleeding effects crippling attributes to balance things out, like the Slow trait, no armor, a low starting health, a low attack, etc.
I hope this was clear and wasn't too wordy! What do you guys think? Would this Bleeding Effect be overpowered or cool and balanced? By the way, I realize this idea is similar to the Burn Effect. That is kinda where I got the idea.