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Races are divided into good and evil.
Evil: Dungeon, Necropolis, Inferno, Stronghold
Good: Haven, Sylvan, Academy, Fortress
If all units in an army belong to the same faction (+1 morale) and are commanded by a hero from that same faction (+1 morale), then the army initially has +2 morale.
For example, if an elven army commanded by a ranger is reinforced with a unit from Haven, the elven army will have +1 morale (its commander +1, its own units +1, a unit from Haven -1), while the unit from Haven will have 0 morale (a foreign commander -1, foreign units, but from the "good" faction, so -1).
If a unit from Haven is added to an army of Dungeon creatures, the Dungeon army will have 0 morale (its commander +1, its own units +1, a unit from the "good" faction -2), and the morale of the unit from Haven will be -3 (a foreign commander -1, foreign units from the "evil" faction -2).