A mountain. Once there was an asura named Krauncha. He was roaming the world when Agastya began praying to Shiva in Kailash. Kaveri was also nearby praying to Shiva. Shiva appeared and asked Agastya what boon he desired. Agastya wanted help in pushing Vindhya down and establishing a holy place in bhuloka. Shiva agreed, made Kaveri a river and placed her in Agastya’s kamandalu to help him establish a theertha. When Agastya was on his way back, Kraucha blocked his way in the form of a mountain and made it rain heavily. Agastya lost his way and roamed the forests for a long while. Finally he understood what happened and with 3-4 drops of kamandalu cursed that Krauncha would remain in that form. He said that Krauncha would get salvation from an arrow shot by Subramanya. As per Skandapurana Asurakanda Krauncha remained in that form from then.
According to Harivansh Chapter 18, Kruancha was the son of Mainaka, who was the son of Mena.
Subramanya was born to Shiva and he fought the asuras. Mahabali’s son Bana hid inside Krauncha. Subramanya used his agni-powered staff to split Krauncha, who then attained salvation as per Mahabharata Shalyaparva Chapter 46. Kalidasa mentions this in Meghdoot and states that swans and vultures flew out of it towards Meru as per Mahabharata Vanaparva Chapter 225 verse 33.
See Parashurama for Parashurama and Krauncha.