If a Hero loses battle, the Hero will be Stunned. In such case the Hero image will turn red and a Skull will appear on it. Hero will not take part in future battles until his Stunning will not pass.

If a Hero wins battle while the mission failed due to some other reason (for example, "Domain Seizure" mission could fail because of the Defensive Buildings that were not destroyed in advance) - the Hero will not be Stunned.

If an assisting Hero loses battle, the hero will stay Stunned in the domain of assistance. Hero will return together with all troops after mission is completed.

Stunned Hero behaves like being in the Sanctuary, but Artefacts on the Hero Doll cannot be replaced.

Stunned Hero stays in the Garrison until player will not relocate the Hero into the sanctuary. Stunned Hero stays in the domain of assistance until the Assistance mission gets withdrawn or completed. Stunned Hero cannot be sent on a mission.
Passive Skills and Spells are not working when the Hero is Stunned.
Mana is not regenerating while the Hero is Stunned.