Death
Birth and death are cyclical, one follow the other. So death is not end of the existence. Freedom from death and birth is Moksha ; human life is meant to achieve this goal
The idea of Punrjanma is available in RigVeda but it is not directly mentioned.
The Kathopanishad is a conversation between Yama and Nachiketa about the secret of life and death.
There was no death in the beginning of creation. Living beings multiplied endlessly on earth. As they had no death, the goddess Earth found their weight too much for her to bear. Afflicted with pain and sorrow Bhumi Devi went to Brahma and prayed for his help. Brahma said that he did not like his creation to find a disastrous end. Brahma consulted with Rudra and Narada to find a way out. At their suggestion Brahma created out of Visvaprakasa (Cosmic Light), a goddess. She was born from the south and Brahma gave her the name Mrthyu. He gave her permission to destroy all the life, which is created by Brahma. When Mrthyu heard that she was to kill living beings, she shed tears and Brahma gathered those tears. She went to Dhenukasrama and other places and performed tapas. At last Brahma called her back and assured her that it was not against Dharma to kill living beings as there is punarjanma to follow and pursue further for Moksha. He changed the tears he had gathered from her face into the various diseases and returned them to her. Brahma also arranged Yama as her companions. Thus the goddess Mrthyu started her dance of destruction.
Sathayvan - Savitri is another story about death. The story is a lengthy conversation between Savitri and Yama. In the end, Sathyavan was restored to life by Yama for the devotion and brilliance of his wife Savitri
MB (Dronaparva #53)