Primarily the Battle of Manzikert (1071) and the civil wars that followed.

Addendum to earlier comment. As noted earlier, most of the elite regiments of the TAGMATA were destroyed at Manzikert (1071) and the Civil Wars that followed. Alexius Commenus lost most of the remainder fighting the NORMANS at the battle of Dyrrhachium (1081). Although the Varangians were also destroyed in this battle - a reconstituted contingent (perhaps from Palace Guards left behind) fought defending Constantinople against the 4th Crusade in 1204.

Best modern source is Warren Treadgold's Byzantium and Its Army 284-1081.