Mana is one of the three main resources in Master of Magic, the management of which is part of the overall strategic challenge. Mana is often confused with Power, even by the game's own documentation and help entries. However, while Power is an intangible magic source that only supplies income for various other assets, Mana is actual, practical energy stored by Wizards in the form of crystals. It is represented both in the game and on the wiki by the icon.
Mana is used mainly for the casting and upkeep of spells, including most Fantastic Units that are created overland using Summoning Spells. Should the need arise, it can also be directly converted to- and from Gold, using Alchemy. This also often serves as the primary source of Mana. While it is also available through the allocation of Power to provide a Mana income, many strategies emphasize using Power to train Spell Casting Skill instead, as that asset is nigh-impossible to raise by other means. Mana is much more abundant, and can be readily found in Treasure, or obtained by conquering the Fortress of a rival Wizard.
Mana has an upper limit of 30,000. Since it does not play any role in the game's final scoring, hoarding it beyond a certain point is generally not wise. Even more so as due to a bug, no player can cast spells during a siege of their capital if they have over 16,383 in the latest official version of the game.
On the other hand, overland spending of Mana is often limited by the previously mentioned Spell Casting Skill. That is, Wizards can only channel as much Mana each turn into casting overland spells, as their Spell Casting Skill. This can be alleviated somewhat by stationing Heroes with the Caster ability in their Fortress garrison. As long as they end their turn there, such Heroes can temporarily add half of the value of their Caster ability to the Wizard's own Skill.
Although they can't cast spells overland, spellcasting units also have their own, replenishing Mana pools. This again, is indicated by the value of their Caster ability, and represented by the Mana icon in combat. They can spend this amount of Mana in every battle, regardless of whether they use it to cast spells or fire Ranged Magical Attacks. Unlike those of Wizards, their spells are not affected by distance modifiers to Casting Costs either, making them ideal choices to extend the player's spellcasting ability.