Lord Ram going back to Vaikuntha after taking Vishnu Avatar:
Lord Rama ruled his kingdom for 11000 years. Once a Sage visited and demanded a private conversation with him. Rama agreed to his demand and asked Lakshmana to guard the door. There was a clause that whoever will try to enter the room during the conversation will get death. The Sage was actually Yama (the God of Death) in disguise. He told Lord Ram that since his purpose on Earth is over after the death of Ravana (demonic king), Kumbhakarna and other demons now he must leave Earth and return to Vaikuntha.
Lakshmana was guarding the door when Sage Durvasa angrily approached and demanded to meet Lord Ram. The Sage threatened that he would curse the city of Ayodhya if he was stopped as well as the entire Raghu Clan (the lineage of the King). So, Lakshmana decided to give up his life for the safety of his kingdom and brother. He entered the room to tell about the arrival of Sage Durvasa and immediately got cursed as per the instructions. Lakshmana quietly went into the Sarayu River and returned to Vaikuntha as Ananta SheshaNag (the Snake with thousand heads). After this Lord Rama too fulfilled his remaining important duties and entered the Sarayu River. Bharat, Shatrughan, Sugriva, Jambavan, Neela, Nala, and all the citizens of Ayodhya along with their wives, servants, mantris, Brahmanas, animals, birds, and all other beings who accompanied him immersed them deep into the sacred river and took the Huge Form of Lord Vishnu.