The skill does what the skill description says: Cast a curse on the target, lowering his stats and HP
Bonus Luk gets reduced to (-1), HP is reduced, and curse is inflicted.
You could inflict coma as well, but as it says: "Has a chance of casting Coma effect on players by a low chance."
The success chance is not for the coma, but rather the skill's success which would be: Cast a curse on the target, lowering his stats and HP