The Pechenegs fought with their neighbors extensively, battling the Kievan Rus for generations. At an important battle between the Byzantine Empire and the Seljuk Turks—the Battle of Manzikert—Pecheneg mercenaries were employed by both sides! (The Turks won.)
In 1091, Byzantine Emperor Alexios I Comnenos, allied with a Cuman army, destroyed the Pechenegs as a nation at the Battle of Levounion in 1091. The Pechenegs had attacked the empire, which had been recently weakened through internal strife. Alexios' victory was the start of a Byzantine revival. The defeated Pechenegs were recruited by Alexios as a military division and settled in Macedonia. Cumans, themselves related to the Pechenegs, attacked their "cousins" in 1094, reducing their numbers.
An uprising of Pechenegs in 1122 was once again defeated by the Byzantines. Over time, those living south of the Danube were absorbed into Romanians and Bulgarians. The last recorded mention of Pechenegs was in 1168. A Byzantine historian referred to Pechenegs as "Chorni Klobuky" (Russian: "black hats"), referring to their national costume. Except for a few names of Pecheneg origin, their national identity has faded away.
So who were the Cumans, and why were they ready to ally with Byzantium against their ethnic cousins? Gold, of course. I'll explain tomorrow.