This Vaisnava festival commemorates the appearance of Lord Nrsimhadeva who has an extraordinary form of half-lion and half-man. This form of Krishna is glorified, described and explained in the Vaisnava scripture Srimad-Bhagavatam.
The story is told how Hiranyakasipu, a powerful enemy of the Lord, sought and gained benedictions tantamount to achieving immortality. He was not to be killed by any man, any animal, or any weapon. He could not be killed indoors or outdoors, in the air or water, or on the ground. He could not be slain during the day or the night and in this way he thought himself invincible.
In a battle with Lord Nrsimha, Hiranyakasipu, met his match. The Lord appeared in His most ferocious feature to defend His dear devotee Prahlada. Hiranyakasipu had been torturing and attempting to kill Prahlada despite him being his son! The Lord does not tolerate mistreatment of His devotee and thus He killed the demon by setting him on his lap, positioning Himself in the doorway at dusk and ripping apart his body with His claws, thus easily maintaining the benedictions given to Hiranyakasipu but effectively dispatching him.
Devotees sing special prayers to Lord Nrsimha invoking His protection from unwanted influences, external and internal.