Even though the Mahabhaya Yakshini has been described on the Internet, I have found that the correct invoking Mantra of this Yakshini has not been given.
The Mantra Sadhana has to be performed under a Smasham Bhoomi or Hindu Crematorium and chanting the Mahabhaya Yakshini Mantra given in this post 1 Lakh[100,000] times.
|| Hreem Mahaabhaya Ehim Swaha||
|| ह्रीं महाभय एहिं स्वाहा ||
Notes- Apart from what is mentioned above, I did not find any other information in the Tantra regarding the Vidhi, which means that the only criteria is the Mantra Chanting This Yakshini Sadhana is for experienced Yakshini or Aghor-Panth Practitioners. As the name Mahabhaya suggests, this Yakshini is said to possess a most terrifying appearance and the Sadhak should not get terrified if she manifests before him and keep his cool otherwise he is likely to end up as a lunatic.
I do not know about anybody who has successfully performed this Mantra Sadhana for Eternal Youth and Immortality.