Sky knight is flying Taros unit, a dragon with a rider on its back.
It is a tier 3 unit which cost 3000.
Among air units, it is the second most powerful, not counting faction specific one at a time units (black dragon, golden dragon, sea dragon, ancient dragon, flying juggernaut).
A group of sky knights together can easily take out any other unit in the game quickly, including defensive structures.
In some matches, Taros players sometimes build up a large number of these, then rush in together to find and destroy the enemy's monarch before they can put up a defense against them.