Wikipedia
Strojimir was the co-ruler of the Serbian Principality alongside his two brothers Mutimir and Gojnik, from ca 851 to his and Gojnik's deposition in the 880s after an unsuccessful coup against the eldest Prince Mutimir (r. 851-891).
He was a younger son of Vlastimir, who ruled in 836–851. Strojimir, together with his brothers Gojnik and Mutimir, defeated the Bulgar Army sent by Boris I, led by his son Vladimir, who, together with 12 boyars was captured by the Serb Army. Peace was subsequently agreed and the two sons of Mutimir; Pribislav and Stefan Mutimirović escorted prisoners towards the border at Rasa. There Boris gave them rich gifts and was given 2 slaves, 2 falcons, two dogs, and 80 furs by Mutimir.
After power struggle between the younger brothers and Mutimir, he and Gojnik was captured and sent as prisoners to Bulgar Khan Boris I in 855–856, as a token of peace-agreement, they both lost their titles as Princes of Serbia and were held at Pliska, the Bulgarian capital. Strojimir was treated well by the Bulgarians, Khan Boris himself chose the wife of Klonimir, the son of Strojimir.
On July 11, 2006, the Serbian government bought a Byzantine-styled seal in solid gold weighing 15,64 g, most likely belonging to Strojimir (Klonimir's father), at an auction in Munich. It was presumably crafted outside Byzantium, and has a Greek inscription (KE BOIΘ CTPOHMIP, "God Help Strojimir") and a patriarchal cross in the centre. It most likely dates to the second half of the 9th century, between 855/56 and 896, when Klonimir tried to take the Serbian throne.