The Mask Laughs is Chapter 10x of Fire Emblem: New Mystery of the Emblem. This chapter is reached if the player beats Chapter 10 in 15 turns or less on Normal Mode (each higher difficulty progressively adds 3 more turns to this limit). Alternatively, the player can fill the requirements by having 3 or more enemy Clerics alive when completing Chapter 10.
Legions will be spawned endlessly, without fail and increasingly each turn after 10. Linger long enough and the whole map will be crawling with Legions, wielding Tomahawks, Devil Axes, Poleaxes, Hammers.
The easy way to determine which Legion is the actual boss is selecting one of the bottom 4 Legions and pressing the L button until the cursor outlines one of the top 3 Legions. The first one that the cursor falls on is the real one. Knowing this, it is entirely possible to complete this chapter in 1 turn.
One farming strategy is to station your units near the main Legion on the top as to be prepared to kill him whenever there spawns more Legions than one can handle.