So, if you gather 25 of these guys, will that provide decent damage over three turns? And will archers lose attack power each turn while shooting from within this thing, or only during the first turn?
So, after three turns, archers will shoot with 1/4 of their original attack power?
Horrible... how do you fight against that?

No, look, instead of lightning, it summons a thunderstorm cloud over an area of 3x3, which lasts for 3 turns. Archers have poor visibility in this cloud, so the damage from ranged attacks in the cloud is reduced. Also, there's lightning (it's still a thunderstorm), so there's damage. The strength of the cloud depends on the number of casters. If you leave the cloud, you won't take damage and you'll shoot normally. But it takes a turn to do that... it's not suitable for archers.
I read it in the manual; it's all clearly described in 350 pages. Familiarize yourself with it before the expansion is released.