Make them randomly occuring on hit, higher probability of a good one happening as your sword rank rises, and more likely on a backstab. Keep it one button that way.
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"Whatever," said Bean, "I was just glad to get out of the toilet."
"Power does not corrupt. Fear corrupts, perhaps the fear of a loss of power."- John Steinbeck
"I'm only acting retarded, what's your excuse?" queried the Gord. - My pet, the Levelup Gnome