Incinerate -Incinerate automatically pierces foes.

Incinerate becomes more powerful if cast continuously from one spot. The increase in damage can happen up to 3 times. At each stage, the flames become visibly larger and will be slightly less accurate, spreading out in a small cone instead of always going in a straight line. The accuracy shift is largely negligible due to the increasing flame width compensating for this. The spell will revert to its first stage if the player stops casting for more than a fraction of a second, even if they do not move from the spot they are standing on.

It's not adding another modifier each stage, it's changing the value of the one modifier the skill has. At the first stage it has 0% more damage. At the second stage it has 100% more damage. At the third stage it has 200% more damage, and at the fourth stage it has 300% more damage.