The tenth King Sri Guru Gobind Singh Sahib Ji was at Sri Anandpur Sahib delivering His Sikhs to Sachkhand. A Gursikh, who would complete his daily prayer of the Panj Granthi (a collection of prayers from Sri Guru Granth Sahib Ji), with love and respect made a mistake. The Gursikh was made to realise that Gurbani is an ang (limb) of Guru Ji and making a mistake while reciting Gurbani is like hurting the Guru. Following this, Pyare Bhai Daya Singh, along with other respected Gursikhs, requested the following to Guru Gobind Singh Ji. "Oh Keeper of the poor! Bless us with the understanding of Gurbani. Without an understanding we don't know whether what we do is correct or incorrect."
Guru Gobind Singh Ji would never return the request of a Gursikh. After all the battles and sacrifices of His beloved family, he left Muktsar Sahib and arrived at Sabo Ki Talwandi. Guru Ji gave the following command to his Gursikhs:-
"Go to Dhir Mal (he was the elder brother of Sri Guru Har Rai Sahib Ji and grandson to Sri Guru Hargobind Sahib Ji) who is at Kartarpur Sahib."
The fifth Guru, Sri Guru Arjan Dev Sahib Ji who has compiled the Aad Granth has left room for the inclusion of the ninth Kings Bani. I will do that and bless you with the understanding of Gurbani." Shaheed Baba Deep Singh Ji along with 24 other Singhs were turned away by Dhir Mal with the following message. "If your Guru is the same roop (form) as the first and fifth Kings, then why doesn't he fashion Gurbani from memory?"
Guru Ji answered this taunt of Dhir Mal in the following way. The true King Sri Guru Gobind Singh Ji gave a great honour to Guru ki Kanshi, Takhat Sri Damdama Sahib, Sabo ki Talwandi. Shaheed Bhai Mani Singh was the scribe as Guru Sahib Ji compiled Sri Guru Granth Sahib Ji from his sacred tongue. There were such great blessings that Jap(u) Ji Sahib, Rehraas Sahib and Kirtan Sohila were written on the first morning. On that evening the Gursikh Sangat listened to the meanings of the completed Gurbani. The understanding of the Gurbani that was written at amrit vela (ambrosial hours of the morning) would be explained in the evening to the 48 Singhs and the rest of the Gursikh Sangat. Shaheed Baba Deep Singh Ji with firm faith would complete the seva of providing the paper, pens and ink. In this way within 9 months and 9 days, on the full moon of Katak, Samat 1762, till until 1763 Bikrami 23 Savan, the understanding of Gurbani was taught. With the inclusion of the ninth Kings Bani, from Ik Oa(n)kaar to At(h)arah Das Bees, Sri Guru Granth Sahib Ji was completed. A great benevolence was done to the world.
After seeing and listening to this limitless, powerful knowledge, the Gursikh Sangat as well the atheists were astonished. By listening to Guru Sahib Ji, the Gursikh Sangat were given the opportunity to rectify their lives. The 48 Singhs gained Brahm Gianta (the knowledge of God) and were liberated while living. The Singhs reached such spiritual states that they lost all care of the world. Seeing this Satguru Ji ordered Shaheed Bhai Mani Singh Ji not to lose care, but to continue in selfless service teaching others what he had learnt. Guru Ji said that he would not forget a thing even if he were to get cut piece by piece. Shaheed Baba Deep Singh Ji was also given this command and was told that he wouldn't forget a thing even if his head were separated from his body.
Both Shaheed Bhai Mani Singh Ji and Baba Deep Singh Ji stayed with Guru Sahib Ji (when They went to Takhat Sachkhand Sri Hazoor Sahib) teaching the understanding of Gurbani to other Gursikhs. Just before Guru Ji ascended to Sachkhand He sent Shaheed Bhai Mani Singh JI to Sri Darbar Sahib Amritsar, and Shaheed Baba Deep Singh Ji to Takhat Sri Damdama Sahib to continue teaching the understanding of Gurbani. Damdami Taksal has been operating under the leadership of Brahm Gianis until now, and will forever more.