Mana domains was a powerful ability in the Realm of spirituality possessed by powerful Tier 4 Emperors. It was a method of using ambient Mana to suppress all hostile forces. Allies on the other hand would receive boons in their basic attributes and combat power as long as the domain is active. This Mana domain is one of the key factors in deducing the strength of an Emperor. A Tier 4 Emperor who had mastered the ability could effortlessly annihilate a group of Emperors who had not. Even a Tier 3 King could clash against a Tier 4 Emperor if he could cast a Mana domain.
The strength of a Mana domain also depends on the mastery of the caster over Mana because some Domain masters could only manipulate ambient Mana a few yards around themselves while some powerful Masters could suppress hostile units within hundreds of meters around them.