Slavers are able to spend their turn raising some kind of slave unit from a province's local populace. This adds unrest and kills province population, and will fail in provinces where the unrest is too high or the population is too low. The kind of slave produced depends on the slaver in question.
Specifically1), if the province's unrest is over 74 + a closed d75, the action fails. Similarly, if the province's population is less than 10 * (49 + a closed d15), the action fails.
The number of slaves produced per turn is an exploding d6 + 4 + the commander's slaver bonus, if any. This quantity is halved, rounded down, if the province's population is less than 10 * (9 + a closed d30). Population loss is calculated as 10 * ((number of slaves + 4)/5, rounded down). The entire process generates 1 unrest per slave found.