Seize is a map objective used throughout the Fire Emblem series
Seize requires that the player capture a castle or a throne on the map to complete it. When a playable character stands on the map tile highlighted and uses the Seize command, the battle immediately concludes, even if all enemies on the map are not defeated.
Typically Seize maps have a Boss defending the spot and will not move from said spot. Some Seize maps allows any unit to seize the specific while others require a specific unit, usually the Lord character.