Activation base is in stamina and values vary with fatigue%.
According to the code in talent_aoe_instant.nss, this talent generates an auto-hit. However, in reality, it sometimes occurs that the game engine would not recognize a target clearly standing inside the AoE, and so the talent will miss. These pseudo-misses are not caused by target's defense rating or dodge chance; they tend to happen whenever there are slight terrain height differences between the character and the target. Such problematic locations are nearly impossible to foresee, but they are consistent and can be avoided (i. e., if you didn't hit your target from a certain position, relocate and retry, instead of trying again from exactly the same location).