Set's Litany is a ring blessed by Set, the spirit of wind. It can be used by either Nils or Ninian in Fire Emblem: Rekka no Ken in order to use a dance or play a song that increases an allies avoid by 10 for one turn. It can be found in the central ruins in chapter 30E/32H: Victory or Death.
It should also be noted, it's Japanese name "Seti's Prayer" is possibly a reference to the crusader Sety (crusader)|Sety]] and/or his holy weapon Holsety (tome)|Holsety]] from Fire Emblem: Seisen no Keifu.
Etymology
In Egyptian mythology Set was the god of deserts, foreigners, and storms. The brother of Osiris, Set accompanied Ra on his ride through the netherworld during the night, fighting off the serpent Apeps. Due to his role as god of foreigners leading to a group of invaders favoring him, Set was later demonized into a god of chaos and evil, who slew Osiris and cut him into pieces and later battled Osiris' son Horus for control of Egypt.