Boniak Khan was a Cuman khan

Khan (title)

Khan is an originally Altaic and subsequently Central Asian title for a sovereign or military ruler, widely used by medieval nomadic Turko-Mongol tribes living to the north of China. 'Khan' is also seen as a title in the Xianbei confederation for their chief between 283 and 289...

 who led the invasions, together with Togortac, on Kievan Rus'

Kievan Rus'

Kievan Rus was a medieval polity in Eastern Europe, from the late 9th to the mid 13th century, when it disintegrated under the pressure of the Mongol invasion of 1237–1240....

. In 1096 Boniak attacked Kiev

Kiev

Kiev or Kyiv is the capital and the largest city of Ukraine, located in the north central part of the country on the Dnieper River. The population as of the 2001 census was 2,611,300. However, higher numbers have been cited in the press....

, plundered the Kiev Monastery of the Caves, and burned down the prince's palace in Berestovo. He was defeated in 1107 by the princes of Rus's forces, near Lubni. He is mentioned for the last time in the Primary Chronicle

Primary Chronicle

The Primary Chronicle , Ruthenian Primary Chronicle or Russian Primary Chronicle, is a history of Kievan Rus' from about 850 to 1110, originally compiled in Kiev about 1113.- Three editions :...

 in 1167, when he was defeated by Prince Oleg I of Chernigov

Oleg I of Chernigov

Oleg Svyatoslavich of Chernigiv , sometimes also styled as of Tmutarakan, was a Rurikid prince whose equivocal adventures ignited political unrest in Kievan Rus at the turn of the 11th and 12th centuries....

.

Boniak is often represented as a sorcerer in Rus' folklore.