Seyyid Kasim Gubari, born Cherif Abdallah in the southeastern Ottoman city of Diyarbakir, began his career as a poet before turning to calligraphy, the craft that made his reputation. Legend attached itself to his skill early: he was said to have written an entire chapter of the Quran, in the deliberately minuscule ghobar style, on a single grain of rice, a story meant to signal exceptional control rather than a claim anyone could verify literally. He earned the honorific Seyyid, marking a claimed direct descent from the Prophet, and became recognized in his own lifetime as one of the finest calligraphers the Ottoman world produced.
His largest and most visible commission was also his last. Sultan Ahmed I engaged him to execute the interior calligraphy of the new imperial mosque being built in Istanbul, including much of the dome and, this atlas's own reading of the sources holds, its monumental roundels. Gubari died in 1615 or 1616, essentially as the mosque itself was nearing completion; construction finished and the building opened for worship in 1616 or 1617.
What survives to be seen today is not, strictly, his own hand. The mosque's calligraphy has been restored more than once across four centuries of continuous use, and Wikipedia's own account of the building notes plainly that the present inscriptions no longer match Gubari's original work exactly. His name survives attached to the commission with more certainty than his actual brushwork does, an ordinary fate for monumental calligraphy exposed to centuries of weather, worship and repair, but one this entry states honestly rather than passing off a restorer's later hand as the master's own.