This is the Samskaram where the person gets the divine Upadesham (dedication) of the Moola Mantra. The Rahasya Traya Mantras are Ashtakshari mantram, Dvaya Mantram and Charma Slokam. A very very fortunate person will get to know these mantras. And the person who is now a Shishya (student) of that Acharyan must not at any cost disclose the Astakshari and Dvayam to anyone. These Sacred Mantras only when got through an Acharya from a true sampradayam and Guru Parampara, will the power of the potency of these mantras.

First of all the most important mantra which is called as the "Moola Mantra" which was originated by Lord Himself. It's the most sacred "Astakshari". This Moola Mantra should be chanted only after taking bath and doing Nitya Anustana and before consuming Prasadam (food).
Prahalaadha, was given Upadesha (initiating) of this moola mantra from Narada Muni even when Prahalaadha was in the womb. Right from the start of the creation this moola mantra is used by countless number of Rishis do this Mantra japam to please Lord Vishnu and to get his blessings.

The second mantra called the Dvaya Mantra. It has two lines and this mantra, too, is Initiated by Acharyan himself. It has to be chanted as many times possible in the persons lifetime. There is no restriction for time and place for chanting this mantra.

5 Yaga Samskaram
1 Thapa Samskara
2 Pundra Samskara
4 Manthra Samskaram