The Upanishads comprise the final Vedas and concern the soul (Atman) and its pursuit of ultimate reality (Brahman). The teachings of the scriptures are both religion and philosophy, and constitute the underlying principles for eastern mystic religions, most notably Hinduism and Buddhism.
Many have used the term transcendence to broadly describe the pursuit of the Atman of its ultimate reality. The Upanishads present the soul as a difficult thing to fully comprehend, but since true knowledge of true self is the underlying principle of enlightenment, a great emphasis is placed on contemplation, introspection and understanding the forces of nature and their effects on the Atman.
The Upanishads are not a singular text, but rather a collection of texts written over the course of hundred years, likely all BC. Their origin as scripture though is far earlier and was passed down as an oral tradition far before.
Even though there are schematic differences between the Western and Eastern approach to religion and philosophy, there are major thematic similarities to be found between the classical Greek philosophers and the writers of the Upanishads. These similarities include the distrust of the physical world and the belief that truth is only partially evident in this space-time.
The authorship of these scriptures is uncertain, as is the exact dates of each particular veda, but likely the texts were originated by the early settlers of Northern India in the Indus River Valley, probably Indo-European travelers or Indo-Iranian travelers.