Once, there was a battle between the demigods and the demons in which the demons were defeated. Being driven from their kingdom, the demons went to the planet, Rasatala, and took shelter of the demon Maya. Maya Danava then created a formidable demon named Tripura and gave him three palaces made of iron, silver and gold. Tripura then proceeded to oppress the inhabitants of the three worlds, so that the demigods became full of anxiety and finally took shelter of Lord Shiva.
After a fierce battle between Lord Shiva and the demons; Maya, the leader of the demons, by the strength of his mystic power, revived the defeated demons by placing them in a well of nectar. After doing this, Maya induced them to again fight with Lord Shiva.
Finally, Lord Shiva took shelter of Sri Krsna, the son of Devaki. Sri Krishna then assumed the form of a cow and Brahma assumed the form of a calf, and they went to Rasatala and drank all the nectar from the well. Shiva then fought with the demon, Tripura. This time, Shiva killed him and thus became famous as Tripurāri. After this incident, the demigods, headed by Indra, glorified Lord Shiva profusely and then returned to heaven with a pacified mind.
From Padma Purāna, we understand from a conversation between Lord Shiva and Parvati that, although Shiva is the incarnation of the mode of ignorance, he is a dear associate of the Supreme Lord. Whenever the Supreme Lord appears in this world to perform His pastimes, His associate, Mahādeva, also appears to take charge of protecting the Lord’s abode. Still, for the pleasure of his devotee, Lord Shiva sometimes doesn’t hesitate to fight with Lord Krsna.
Lord Shiva becomes very easily pleased, and that is why he is known as Āshutosha.