Dark Sun: Seared to the bone
If you roll a natural 20 with a defiling attack, the following conditions apply:
Not only do you max out the damage (per the standard rules) you increase the damage by xd6+5 per tier (Heroic 1d6+5, paragon 2d6+10, Epic 3d6+15).
Each ally within the defiling range of you takes his FULL healing surge value in damage. This damage ignores immunities and can cannot be reduced in any way. Additionally, the damage to the world extends out a number of squares equal to the level of the power you are using (instead of half the level of the power).