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If a unit is unique only one instance of it exists in the game. This has a few implications:

• If they're already on the strategic map, then they cannot be Summoned.
• If they're already on the strategic map as a commander, then Wishing for them teleports them to the province, starting a defense battle.
• If they're already on the strategic map as a unit, then Wishing for them steals them from their owner.
• If they're dead, then Summoning them resurrects them.
  • Empowerments and most Afflictions will remain, but the unit's shape and Commander status are determined by the spell.

Most Heroes and many national summons are unique.

Notable unique units include the Elemental Royalty, the Treelords, the Eater of the Dead, the Ice Devils, the Arch Devils, the Heliophagi, the Demon Lords and the Doom Horrors.