Treachery (悪逆) is Chapter 26 of Fire Emblem: Fates in the Conquest Version.
Subjective: The following part of this article is based upon the editor's personal experiences and opinions, and therefore may not be applicable for all readers.
Your units start off at the top of the map, while the area to seize is occupied by Iago in the middle of the room. There are several doors blocking the way to him, so a Locktouch unit will be needed to open them. The first room entered will contain Heros, which shouldn't be much of a threat as they can be picked off easily. Once that room is cleared, there are two doors, one on either side that can be opened. The left side will contain Faceless and Golems, while the other side will have Sorcerers and Maids. If your units are physically defensive, they should take the left side while magically defensive units should take the right side. Both sides also have on chest, and should be looted before leaving. The bottom room contains both Generals and Berserkers (including Ganz), who will rush over to the closest side players are entering from.
Once they are cleared, open the door leading to Iago. Once you defeat him, remember to loot the three chests near him before seizing.
Prince Ryoma of Hoshido
The Hollow King