Black Flames (黒き炎 Kuroki honō Black Flame in the Japanese Version) is Chapter 17 of Fire Emblem Fates in the Revelation Version.
This chapter begins immediately after the conclusion of Chapter 16; thus, the player cannot exit the battle, save prior to starting it, or advance any supports. The player can, however, move inventories around and reselect units. Any unit hurt in the previous chapter will not retain their injuries, giving the player a complete fresh start for this map.
It may be tempting to divide your units into two groups on this map, which will make clearing the chapter faster. In lunatic, however this could increase the risk of losing a unit substantially and may result in the player restarting from chapter 16 again. To mitigate this risk, keep all units together.
Odin, Niles, Laslow and Peri may be a liability on this chapter due to their low level, especially in Classic Mode (though this can also be annoying in Casual, and sometimes even in Phoenix Mode). However, ensure to keep them with the group as you do not want them to be cut down by enemy reinforcements, and an astute enough player may even be able to level them up enough for promotion. Pairing them up with other characters might be a solution.
The cavaliers, paladins and great knights on the right side of the map will charge towards you from turn 1. Ensure to have your highest defence characters lead the front line. It may be safer to pair them with other units in defensive stance to ensure their survival. Once these enemy units are taken care of, advance carefully to the boss to end the chapter. Keep in mind that enemy reinforcements will pour out of the enemy strongholds when only when a playable unit enters a certain part of the map. So caution should be excercised when advancing.
There are two strategies to approach this map. One is to unite Xander's and Leo's forces to the main army as soon as possible, and the other one is to use them to advance downwards from the upper right corner of the map. Both tactics have their own merits and shortcomings. Uniting the army may look safer and gives you the option of interchange units between two fronts while also allowing Xander's team access to healers. However, it also means you will be playing in pursuit for a few turns and even if you manage to unite with the main army, they will be stuck in a pincer attack between cavaliers and knights, which might not be favorable if the main army lacks manpower.
Advancing Xander's troops down the right side of the map is recommended on normal difficulty because the horsemen are few with sparse formation (compared to the knights on the left) which is easier to pick apart. The right side of the map also have a fair composition of plains and forest terrains which you can use for Elbow Room and cover respectively according to your needs. However, advancing on two fronts means Xander's team will not have access to any healer and their retainers might see little use without healers. Also a problem that few people address is focus. Advancing on two fronts sometimes can cost too much of your attention and you will be more likely to make tiny mistakes such as positioning units on the wrong side of the enemy or putting a unit on a spot that's exposed to attacks from multiple enemies, which happens easily if you overestimate Leo's Brynhildr.
Regardless of the approach you take, make sure to use your newly acquired big brothers to bait the enemy. Ryoma can act as an evasive tank while Xander's defense can shrug off most attacks on this map. Worrying about EXP distribution might cost you this chapter as well as the previous one so keep this in mind. Both the knights and cavaliers advance in groups of 3-4, so make sure you stockpile your units to intercept. A favorable move would be using your ranged units to soften the front and using your melee units to deal the finishing blow from up close to slowly advance while reconstituting your (hopefully durable) front line.
The ballista and magic orb are not to be taken lightly also. The magic orb would be the main army's first priority as it's area attack can cost you dearly. Flying units and any non-tanky units should be kept well away from the ballista reach because they are more effective than what you might initially think. Use the forts to safely advance your big brothers and take the Bow Knights out as soon as possible. Everything in this chapter boils down to advancing slowly and capturing the ballista, after which the chapter is effectively won because the Generals and Iago can't provide as much threat as the rest of the map.
The script for this chapter can be found here.