from the Synaxarion:
The Monk Feodor (Theodore), Prince of Ostrozh, gained fame with the construction of churches and by his defense of Orthodoxy in Volynia against the enroachment of Papism. He was descended from the lineage of holy Equal-to-the-Apostles Vladimir (Comm. 15 July), through a great-grandson Svyatopolk-Michael, prince of Turov (1080-1093) and later GreatPrince of Kiev (+ 1113). The first time the name of the Monk-prince Feodor is mentioned is under the year 1386, when the Polish king Jagiello and the Lithuanian prince Vitovt affirmed for him hereditary possession -- of the Ostrozh district and they augmented the Zaslavsk and Koretsk surroundings. In 1410 the Monk-prince Feodor participated in the defeat of the Teutonic Knights of the Catholic Order at the Battle of Gruenwald. In 1422 the holy prince, because of sympathy to the Orthodox in Bohemia, supported the Hussites in their struggle with the German emperor Sigismund. (The holy prince introduced into Russian military arts a particular tactic -- the Hussite formation, i.e. the Taborite, adopted by the Ukrainian Cossacks). In 1432, having gained a series of victories over the Polish forces, Saint Feodor compelled prince Jagiello to protect by law the freedom of Orthodoxy in Volynia. Prince Svidrigailo, having become apprehensive of the strengthening of his ally, locked the Monk Feodor into prison, but the people loving the saint rose up in rebellion, and he was freed. The Monk Feodor was reconciled with the offender and presented himself to him for help in the struggle with the Lithuanian-Polish parties. In 1438 the holy prince participated in a battle with the Tatars. In 1440 with the entering upon the Polish throne of Cazimir, -- youngest son of prince Jagiello, Saint Feodor received the rights of administration of the city of Vladimir, Dubno, Ostrog, and became possessor of extended holdings of the best regions of Podolia and Volynia. All this together with princely power and fame the Monk Feodor left behind, having entered after 1441 the Kievo-Pechersk monastery where, -- having taken on monasticism with the name Feodosii (Theodosii), he pursued asceticism for the salvation of his soul until the time of his repose to God. The year of repose of the Monk Feodor is unknown, but it is without doubt, that he died in the second half of the XV Century. In extreme old age (S. M. Solov'ev in his "History of Russia" reckons the year of his death as 1483). The monk was buried in the Farther Caves of the Monk Feodosii (the Comm. of Sobor/Assemblage of the Monastic Fathers of the Farther Caves is 28 August). The glorification, apparently, was at the end of the XVI Century, since in the year 1638 the priestmonk Athanasii Kal'nophysky testified, that "the Monk Feodor rests in the Theodosiev Cave discovered whole in body".