CRIT: # effect would be very useful for 4e
Examples:
Biting Volley power: Each attack can score a critical hit on a roll of 18-20.
Sunder Armor: Until the end of Gharash Vren's next turn, any attack roll against the target can score a critical hit on a roll of 18-20.
I am sure there are others...
I could attempt an extension but maybe the Devs might think its a good idea to have it in the core 4e ruleset :-)
