From Worms Knowledge Base
When selected, your worm is given the opportunity to call in a lightning bolt from the heavens. Simply select a position on the map and a grey cloud will form in the sky above... then a crack of thunder will follow a bright flash as a lightning bolt strikes the land, or unfortunate worm, directly below it.
Alternatively, to prevent the weapon from becoming too deadly accurate, the lightning could autonomously strike the highest piece of land on the terrain, or land nearest to the cloud. In any case, the strike would create a small but intensely powerful explosion, causing high damage to any worms in the immediate vicinity.
Like any strike, this is useful for those situations when you aren't really in any position to use a handheld weapon. In fairness it isn't that different from the Airstrike, but is more concentrated and more accurate.
Finally, the ability to control the elements. This weapon is a must for Worms - crazy in it's application, but realistic in its existance. And the animation for a worm being struck by lighting is just asking for a skeleton - even if worms don't have them.