From the description of Silanna: "...Silanna's Breath is a cloud of stone shards that either tear the target to pieces or completely cover it, turning it into a stone statue."
I have an idea to represent this ability as an alt-upgrade skill for green dragons (in my opinion, prismatic breath lightning doesn't fit because lightning is air, not earth).
The ability could be called "Petrifying Breath," and in terms of mechanics, I'm thinking of something similar to the medusas from Heroes 4 (they had a similar ability). In Heroes 5, it could work like this: the first unit in the stack turns to stone. "Cleaving Sword" on a dragon looks a bit strange – what do you think?
Source: http://ru.mightandmagic.wikia.com/wiki/Силанна
Блистает там воздушностью прогалин.
Но между кедров, полных тишиной,
Расщелина по склону ниспадала.
О, никогда под бледною луной
Так пышен не был тот уют лесной...