John I Tzimiskes
The designation "Immortal" to describe a military unit was used twice during the Byzantine Empire, first as elite heavy cavalry under John I Tzimiskes (r. 969–976) and then later by Nikephoritzes, the chief minister of Emperor Michael VII (r. 1071–1081), as the core of a new central field army, following the disastrous defeat of Manzikert by the Seljuk Turks in 1071.