May be in the 7th or 8th campaign, the heroes Arkantos, Ajax & Chiron have to
find three relics and must safely return to the three temples on their way 
they meet shades (A kind of unit which is invisible to enemy units) however,
they cannot attack buildings but they are very effective against myth units.
Just click on the shade and select any enemy myth unit e.g.:(Colossus, Hydra,
Kracken) to attack them. The shades grab the enemy myth unit souls and diminish
away. (They can be used only once against one myth unit). Dont use shades on 
ineffective myth units like(Minotaur,Cyclops). Use them wisely they will 
surely sustain you. If more doubts persists, Mail me.