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 highlighted tile and uses the Seize command, the battle immediately concludes, even if there are still enemy units present.
Typically, Seize maps have a Boss defending the spot and will not move from said spot. Some Seize maps allow any unit to seize the spot, while others require a specific unit, usually the Lord character.