Atma Sakshatkara means “witnessing of the Atman” (i.e. self-realization), and is the name of a chapter within the jnana-pada (knowledge section) of the Sarvajnanottara Agama, an obscure Upagama (subsidiary Agama) text. The sublime non-dual teaching of this text is in the form of a dialogue between Siva and Guha (Kartikeya-Murugan), where Lord Siva is the supreme Guru, and Guha is the Disciple.
This text was discovered by Sri Ramana Maharshi, who translated the chapter entitled Atma Sakshatkara into Tamil sometime in the early 20th century, and also composed the introductory benediction verse. This English translation of that chapter is done by Dr. H. Ramamurthy.