An Undying Oath is the Final Chapter of Fire Emblem: Thracia 776.
This chapter can be either extremely easy, or a nightmare, depending on how you have prepared. Sleep staves, Warp Staves, Rewarp staves, Rescue staves, Tina's Unlock staff and Door Keys make this chapter a lot easier. The former for nullifying most threats on this map, the middle three for mobilizing your units and shuffling them from an undesired unit formation, and the latter two to allow you to enter the Deadlords' rooms.
For units, there is no question. Bring your strongest units, A-rank staff users, or any with Leadership stars, for extra hit and avoid to make the chapter more reliable, and the fatigue mechanic does not matter, as this is the final chapter.
First, Sleeping the Deadlords should be your priority. Canis can only be slept by use of the Blizzard tome, where as most of the others can be slept by Sleep staves (Although, keep in mind they are on the Magic floor tiles). If you are able to remove any (Preferably Canis due to her Berserk Staff) on turn one reliably, you should try to remove them.
Players should be cautious when opening the doors, as the enemies in some are extremely powerful. If the Deadlords are asleep, all you have to do is have a unit with superior build capture them, then stand on their tile. When all 6 tiles have been stood on, Veld will appear in the center room. Since there is a spot where a two range unit can attack him from, you can just warp a two range unit to kill him, then Warp Leif to complete the game.