Shoots thorn of hell to an enemy in the rear and deals magic damage equal to (50/120/250)% of the Hero's attack power. A flame stack will pile up equal to (1/2/3).
An enemy damaged by Thorns of Hell is lit on the fire and receives neutral damage equal to 50% of the hero's attack. When this effect is stacked 4 times, the hero shoots a flame arrow that deals magic damage equal to 350% of the hero's attack and 150% neutral damage. (Active once every 3 seconds)
The in-game passive description has a typo; Her ultimate flame arrow attack activates at 3 stacks, not 4. Additionally, the flame stacks don't have to be all stacked on the same target, nor do they have to be stacked all at once. Similar to Leon's passive, it can be spread out between waves, and when the final stack is stacked, the Ultimate Flame Arrow will activate.