Nagarjuna

It is said that the life span of Nagarjuna was attached to the King Udayibhadra. This king had a son, Kumara Shaktiman and he wanted to be the king. The king's wife, Kumara's mother, had instructed Kumara that in order to become king, he would need to cut Nagarjuna's head off. The mother also said that since Nagarjuna is such a compassionate man that surely he would agree. Amazingly, Nagarjuna agreed. However Kumara couldn't cut off his head with a sword. Nagarjuna said that in a previous life he had killed an ant while cutting grass. Therefore, due to karma, only the blade of kusha grass could cut Nagarjuna. Kumara did this and was able to behead Nagarjuna. Strangely enough upon his supposed death, the blood coming from the severed head turned into milk and the head spoke, " Now, I will go to Sukhavati but I will enter this body once again." Sukhavati is the western pure land of the Buddha Amitabha. Sukhavati translates to mean, "Land of Bliss" Upon this revelation from the talking head, Kumara separated the head from the body as far as possible. However, legend states that every year the head and body become closer and closer to each other. When the head joins with the body, it is said that Nagarjuna will return to this world to walk again.
— Dennis Waller, in the introduction to “Nagarjuna's Tree of Wisdom”
There's another (similar) version of the story here: https://buddhiststories.wordpress.com/…/how-did-nagarjuna-…/