The Lair of Fire (Men and Monsters in the European version) is Chapter 20x of Fire Emblem: Shadow Dragon.
To advance to this chapter, complete Chapter 20: Camus the Sable with 15 or fewer units alive. Ymir will appear at the beginning of the player's turn at the upper right corner and can be recruited by talking to him with Marth, then he will leave the map. After that, seize and he will be available next chapter. In the chests you can find: a Wo Dao, a Longbow, a Poleaxe, a Killer Bow, a Killer Lance and a Killer Axe. Also, be sure to bring a thief in this chapter to open the chests, or Marth using the Fire Emblem. The boss should not be too hard to defeat, despite being a general.
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.
Talk to Ymir with Marth and he will leave, when you complete the chapter, Ymir will join your army. Do not forget to get all of the treasure chests in this chapter. Get all of the treasure and fight Larissa. The next Gaiden Chapter is Chapter 24x, so if the player desires to access it, have Tiki die and have no Falchion.