An attack roll is considered a Critical Strike in the following circumstances:

Roll d% and consult the appropriate Critical chart for the damage type. On an F+ severity, use the same result on both the 'E' column and the 'A' or 'B' secondary column.
The severity of the Critical depends on the amount of damage:

Table: Critical Severity by Damage
  Crit Level    HP Damage 
  A 
1-2
  B 
3-5
  C 
6-9
  D 
10-14
  E 
15-20
  F / E+A 
21-27
  G / E+B 
28-35
  H / E+C 
36-44
  I / E+C+A 
45-54
  J / E+C+B 
55+


If the attacker is smaller than the target, shift the Critical Level up one row for each size category difference to a minimum of 'A.'
If the attacker is larger than the target, shift the Critical Level down one row for each size category difference to a maximum of 'J.'

Critical Results
The critical charts result in a number of different effects. Remember to divide all dice penalties expressed in d100 terms by 5 to get their d20 equivalents.
Note that all penalties stack.

For example, Jozan bashes an evil cultist with his mace for 8 points of damage. It handily drops the cultist to 0 hp, so the result is a Critical Strike of 'D' severity. Jozan's player rolls d% and gets 27. We consult the Krush critical chart: "Strike to the belly makes foe regurgitate last meal." The cultist gets sick and collapses.

Alhandra casts Smite Undead II, then chops at a ghoul with her longsword and rolls 19 to hit against AC 12. A Critical Strike! She rolls low for 6 points of damage, a 'C' Slash critical, but she makes 78 on her d% roll. The critical chart reads "Hard chop cracks sternum." It deals 12 extra damage, 2 bleed / turn, a -4 injury penalty from the broken bone, a -2 fatigue penalty, stagger, and 5' knock back. The spell effect adds a 'B' Holy critical, which reads "Tiny balls of light radiate out from the blow, burning radiating lines across foe's chest." This adds 34 extra damage, stunned for one turn at -10, knocked down, staggered, and a breakage roll for the ghoul's armor or weapon.
The final blow deals 52 points of damage with a broken bone, inflicts 2 bleed / turn and -6 in ongoing penalties, staggers, knocks down, and knocks back the ghoul 5 feet, and stuns it for one turn at a -10 penalty. If the ghoul were wearing a breastplate, it'd be toast. That ghoul is more than dead.

Mialee casts Ray of Frost on a goblin brute, rolling 23 against the foe's AC 16. A Critical Strike! She maxes the damage for 8 points, a 'C' critical, and since she's one size category larger than the goblin, it bumps up to a 'D' critical. Mialee rolls 94 on d%! "Foe's legs are engulfed in cold. Frost crisscrosses foe's extremities. Core temperature drops. In 9 rounds, foe dies." The hit results in death, deals an extra 3 damage, causes a -8 wound penalty from frostbite, stuns the target for 3 turns at a -10 penalty, and then the target remains stunned for 6 turns with a -5 penalty. After that, the death timer trips and the foe dies. Unless this brute is a whole lot of trouble, ze's pretty much down for the count.

An enemy priest casts Sacred Flame on Kal, who flubs his Dexterity save with a 6 against DC 13. He takes 2 points of damage, an 'A' Holy critical. The DM rolls 61 on d%: "Strike to the shoulder sends burning pain running down foe's arm to fingertips." The spell deals 14 extra damage and inflicts a -10 stun for 1 turn. Kal groans as the spell wreaks its terrible effect for 16 points of damage and goes on the defensive for a turn to quaff a potion.

Lidda foot pads behind a half-orc guard and whacks him in the head with her sap, going for a Subdual Critical. Even with the -5 penalty to hir attack, she still hits the unsuspecting foe for a Critical Strike. With Sneak Attack, she rolls 10 points of damage: a 'D' Critical. However, since Lidda is a small creature and the half-orc is Medium-sized, her Critical severity is reduced by one level to 'C.' She rolls 42 on d% and consults the chart. The highest Head result up to 42 is 20: "Hard bonk to head causes foe to bite tongue and mouth fills with blood. Foe is unable to speak or scream for 2 rounds." The critical deals 4 extra damage and stuns the guard for one turn at a -5 penalty. Disoriented as he is, Lidda should be able to finish knocking the guard out before he can recover his voice.

Ember is trying to subdue her foe, a gnome infiltrator, before he can get away. She goes for a Grappling Critical. It's close, but she still manages to hit with a -5 to her attack roll: a Critical Strike. She rolls low for 2 damage, an 'A' critical, but since she's larger than the gnome, it bumps the level up to 'B.' She rolls d% for 66. "You tackle foe around waist, overwhelming him." It inflicts 10 extra damage, yields a -20 grapple result, stuns the target with a -15 penalty, staggers hir, and knocks hir back 5 feet. Good grapple, Ember!


*Critical Table results from Rolemaster Unified Core Law, S. Charlton and P. Fenlon, 2023 by Iron Crown Enterprises Ltd.