There’s a certain elegance in letting the attack roll also equal the severity of the blow. In other words, the higher the roll, the more likely that a successful attack is a critical hit. OD&D has this buried in the monster descriptions under Purple Worm:
Any hit which scores over 20% of the minimum total
required to hit, or 100% in any case, indicates the
Purple Worm has swallowed its victim.
In other words, four points above the target number, or a natural 20, is a critical hit with a special effect (swallowed.) The Greyhawk supplement uses the same mechanic for the Sword of Sharpness, with a critical hit with the special effect “Sever Limb or Head”.
This is a kind of clunky mechanic, especially for either the THAC0 or d20 System approach, but a d20 roll-under system would simplify it:
- Add Descending AC and either fighter level or monster hit dice together to get a target number.
- Roll 1d20. If result <= target number, attack is a success.
- Result rolled, if successful, is the severity.
- Severity 4+ is a critical hit with a special effect.
Delta’s Target 20 System can also work, but the last digit of a successful result (20+) is the severity, with the understanding that any result of 30 or more is Severity 9. Again, Severity 4+ is a critical hit with a special effect.
But what are those special effects? That’s something I plan to expand on in a follow-up post.
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