Demon Kalanemi was deputed by Ravana to kill or obstruct Hanuman who was on his way to Himalayas to bring Mrita Sanjeevani (medicinal herbs to rescue Lakshman).
Kalanemi found the orders of Ravana against the rules of battle. He objected to it as it was cowardice and cheating. But Ravana did not agree and Kalanemi thought that better than serving Ravana it was to get killed at the hands of a Sri Ram Bhakt.
Kalanemi sets off for the Himalayas and produces a lake, a temple, a garden and he himself appears in the garb of an ascetic. Hanuman who passes through the area asks the ascetic for permission to quench his thirst.
Hanuman enters the lake, where an alligator, an Apsara (divine being) under a curse, takes hold of his foot.
Hanuman easily overpowers and kills the alligator. The Apsara is released of the curse and informs Hanuman that the ascetic is a demon in disguise.
Hanuman then slays Kalanemi and the demon dies remembering Sri Ram’s name.