Wikipedia based on academic sources https://en.wikipedia.org/wiki/Wikipedia:Attribution + https://en.wikipedia.org/wiki/Wikipedia:No_original_research, and not modern Photoshopped fan art flags, Wikipedia is not "Game of Thrones".
Description: "The flag of territory of aproximate present-day Slovakia from the 14th century. There is a golden doublecross at the golden triple-hill in the red field. The flag was created in the 1301, during the rule of the Louis V., whose protage was the ruler of upper part of the kingdom, Matúš Čák Trenčianský and this flag was de facto symbol of Matúš's territory."
Who is "Louis V."??? A fantasy king? https://en.wikipedia.org/wiki/List_of_Hungarian_monarchs
Stephen V of Hungary (1270-72) already used the same coat of arms:
https://static-cdn.arcanum.com/nfo-resources/magyarnemzettortenete_pic/magyarnemzettortenete/szb-02-315.jpg
https://historia-cronologia.lapunk.hu/kepek/v__istvan_fuggopecsetjenek_hatlapja.jpg
Ladislaus IV of Hungary (1272-90) also:
https://en.wikipedia.org/wiki/File:Coa_Hungary_Country_History_Ladislaus_IV_(1262-1290).svg
https://commons.wikimedia.org/wiki/File:Ladislav4_1.jpg
I assume this flag based on a this picture from the Chronicon Pictum: https://commons.wikimedia.org/wiki/File:Rozgony_Battle.jpg
We can see exactly, in this picture the Hungarian king use that flag at battle of Rozgony, the dead soldier is from the army of the king. https://en.wikipedia.org/wiki/Battle_of_Rozgony
Even we can see exactly the same symbol (double cross+trimount) on the body of the Hungarian king. King Charles I fought a lot for the Hungarian throne, he emphasized his connection to the royal Hungarian Arpad family by materline line, that is why another flag shows his Anjou lilly coat of arms with the Arpad striped.
Morover the Chronicon Pictum clearly say the fallen flag is the royal flag: "Gyürke was the king’s standard-bearer, and when he was killed and the royal standard fell, the king fought under the standard of the crusaders."
Even the Hungarian coat of arms includes the Arpad family and Hungarian double cross symbol together: https://en.wikipedia.org/wiki/Coat_of_arms_of_Hungary
His son Louis used again the same double cross coat of arms as previous Arpad kings: King Louis I of Hungary (1342-1382): https://en.wikipedia.org/wiki/File:I._Nagy_Lajos_király_(1342-1382)_második_kettős_felségpecsétjének_hátlapja_(1366-1382).jpg
Bela IV of Hungary (1235–1270): https://en.wikipedia.org/wiki/File:IV._Béla_aranypecsétje.jpg
Morover we can see exactly the same flag in the same book on the other Hungarian images: https://en.wikipedia.org/wiki/Chronicon_Pictum
Examples:
https://commons.wikimedia.org/wiki/File:Istvan-ChroniconPictum.jpg
https://commons.wikimedia.org/wiki/File:Stephen_I_intercepts_Gyula_(Chronicon_Pictum_040).jpg
https://commons.wikimedia.org/wiki/File:Stephen_I_defeats_Kean_(Chronicon_Pictum_041).jpg
https://commons.wikimedia.org/wiki/File:Andrew_II_of_Hungary.jpg
https://commons.wikimedia.org/wiki/File:Chronicon_Pictum_P129_IV._László_és_gyilkosai.JPG