Oath of the Dagger is the tenth chapter of the Azure Moon route and its Endgame, accessed when Byleth chooses the Blue Lions house.
Having seized Enbarr, Dimitri, Byleth, and the Blue Lions storm the castle where Edelgard awaits. Willing to use any means to win the war, she unleashes the power of her crests, transforming into a Demonic Beast of unparalleled power, but maintains full control over her monstrous form.
The battle is long fought and hard, but ultimately Dimitri emerges victorious over Edelgard, causing her form to dissipate. Dimitri offers one last chance for Edelgard to submit peacefully, but she instead throws the dagger Dimitri returned to her, prompting him to kill Edelgard as the dagger pierces his shoulder.
With the death of the Emperor, peace is restored across Fódlan as the Adrestian Empire is merged into the Holy Kingdom of Faerghus. With all of Fódlan united under the rule of Faerghus, Dimitri becomes the first king of the united continent. Rhea is rescued from imprisonment, but is too weak, forcing her to step down as Archbishop of the Church of Seiros. Byleth takes over the mantle and together with King Dimitri, peace is restored.
If Byleth chose a partner that they wish to spend the rest of their life with, they will meet at the Goddess Tower and the two profess their love (or establish a platonic, but eternal bond of friendship in the case of a Male Byleth with Alois/Gilbert). The two will spend the rest of their lives together helping to spread peace throughout Fódlan
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.
This is the final battle of the Azure Moon map so do not be stingy with any remaining resource available including funds and forging materials. It is better to deplete your funds and forging material to ensure that your units are absolutely prepared for this battle as it mirrors the previous map. Nothing aside from Renown carries over so splurge on whatever you deem necessary to complete this map. As the map is the final one, do not worry about Weapon Usage as the weapons will also not transfer either so use them as you see fit to push your way to the final goal of defeating Edelgard.
The map is filled with high ranged enemies using a Fire Orb on the left side of the map and a Balista on the right side of the map. as well as a few Gremory using Meteor. These are usually the most problematic enemies and should be dispatched as soon as possible. Killing the former two as soon as possible and baiting out the latter is advised much like the previous map. The other problem on the map is Edelgard herself. She will constantly be assailing your allies throughout the battle with a massive ranged assault. She will launch two of these attacks each turn, however she will not target the same unit twice in the same turn. It is worth noting will always target Dimitri first if he is in her range.
The player has three routes to get to Edelgard either charging the middle, left, or right side of the map. Dividing the army in to two to charge the side routes then converging the middle or having all rush one side are solid strategies to effectively split and conquer the sides quickly or combine and conquer the routes slowly, but with more unit options when taking down a side. Regardless of choice of strategy dealing out the nearby starting rooms of the routes on the left and right will provide some chests with useful items on the left while the right has a mage that is commanding several powerful mages throughout the battle field. He is deep in the room and also carries Bohr Χ which will reduce your unit's HP to 1 if it hits and can attack any ally unit within 3-10 spaces. He will not move from his starting spot and the spell could be evaded with a character with high enough Speed and Dexterity like a Falcon Knight Ingrid. He can also only cast it three times, thus if the player is lucky, he can waste its uses and allow the player to proceed normally. One could also have Flayn use Rescue to position someone with high enough movement such as a Paladin or Falcon Knight into enough movement range in order to attack him as he cannot attack from 1-2 range. The risk from taking him out early is rewarding as upon his death, several powerful mages immediately depart the battlefield, taking away from the enemy's overall forces.
Once the player has reached the final chamber with Edelgard with Dimitri, the battle is straightforward assuming all other enemies are dealt with. She loses her massive range, instead reducing it to 5 tiles. This does mean that even a Longbow Curved Shot user will be hit by her attacks. She has four health bars, which means that she will not go down easily. As mentioned above, being stingy with your weapons uses are no longer advised and any means to severely reduce her health is advised. However, it should be noted that she will gain stats and more maximum health with each bar depletion. Gambits can help to shatter her singular shielded tile, but may help open the door for devastating damage if you are able to break them. Mercedes and Flayn if one or both are deployed become useful in this situation, and in the map in general, as Fortify provides massive, widespread healing and can do so safely out of her range. Edelgard will occasionally use a huge Area of Effect gambit like attack around her, making the Faith spell even more useful. Otherwise, Mercedes has Physic for a single target ranged Faith healing while Flayn can Rescue a vulnerable target.