Only Rogues (All paths of the Rogues), some Rangers and a few Nobles might learn this skill.
It is a passive skill, that can be upgraded up to the maximum of three times.
The first level would grant you about 4% chance of poisoning your enemy to lose 3hp every third second.
So it would look like this:
1st Level
4% chance of infection, 3HP draining pr. 3rd sec.
2nd Level
7%chance of infection, 4HP draining pr. 3rd sec.
3rd Level
10% chance of infection, 5HP draining pr. 2nd sec.
No matter what state of the skill that you have learned, the poisoning should wear off after 15-20 seconds, just to balance the otherwise over-powered skill.
I also think that almost every race with a hero that is either a Rogue, Ranger or Noble, should be able to acquire this ability. Although, only humanoids, because the user of this skill, should obviously be wielding a blade of some sort. Or at least a piercing weapon as either a Dagger, Knife, all Swords, Polearms and Spears.