Khan of Heaven

Khan of Heaven or Tian Kehan, Celestial Khan, Heavenly Khan, Tengri Khan was a title addressed to the Emperor [Taizong of Tang] by various Turkic nomads. It was first mentioned in accounts on May 20, 630 and again on October 24, 646, shortly after the Emperor Taizong's campaign against [Eastern Tujue|Eastern Turkic Khaganate] and Xueyantuo were annihilated by the Tang dynasty.
The title Tengri Khagan also used to refer other Turkic rulers, both known as the Tengri Khagan or Täŋridä qaγan to the Chinese, during the Second Eastern Turkic Khaganate and Uyghur Khaganate periods.
The title remained in use by Taizong's successors until 779 AD, including Wu Zetian who assumed the title along with also the masculine form of the title emperor from 685 to 705.
To name a few instances of the title "Khan of Heaven" in use, two appeal letters from the Turkic hybrid rulers, Ashina Qutluγ Ton Tardu in 727, the Yabgu of Tokharistan, and Yina Tudun Qule in 741, the king of Tashkent, addressed the Emperor Xuanzong of Tang as Tian Kehan during the Umayyad expansion.
A later letter sent by the Tang court to the Yenisei Kirghiz Qaghan explained that "the peoples of the northwest" had requested Emperor Taizong of Tang to become the "Heavenly Qaghan".