Andrew III made his maternal uncle, Albertino Morosini, Duke of Slavonia, in July 1299, stirring up the Slavonian and Croatian noblemen to revolt.
A powerful Croatian baron, Paul Šubić, sent his brother, George, to Italy in early 1300 to convince Charles II of Naples to send his grandson to Hungary to claim the throne in person.
The king accepted the proposal and the twelve-year-old Charles arrived to Hungary with his escort.
They landed at Split in Dalmatia in August 1300.
From Split, Paul Šubić escorted him to Zagreb, where Ugrin Csák swore loyalty to Charles, who also handed over Požega Castle to him.
Ugrin placed the young pretender under his guardianship.
Other sources confirm that he already took an oath of allegiance to the pretender prior to that.
According to the Illuminated Chronicle, Ugrin was one of those lords, who requested Pope Boniface VIII to provide a new king for Hungary.
In February 1300, Charles II listed Ugrin's name among those Hungarian barons, who urged him to send his grandson to Hungary.
His familiaris and skilled diplomat, Benedict also visited the Neapolitan court several times in that period.
It is possible that Ugrin turned against Andrew already in 1298, as he did not countersign the decrees of the national diet in that year, which aimed to destroy forts built without permission and ordered the punishment of those who had seized landed property with force.
Ugrin did present neither before the king's judicial court in 1300, during a lawsuit regarding his former palace in Esztergom.

Andrew III died on 14 January 1301.
With his death, the House of Árpád became extinct.
A civil war between various claimants to the throne – Charles of Anjou, Wenceslaus of Bohemia, and Otto of Bavaria – followed Andrew's death and lasted for seven years.
Hungary had disintegrated into about a dozen independent provinces, each ruled by a powerful lord, or oligarch.
Among them, Ugrin's distant relative, Matthew III Csák dominated the northwestern parts of Hungary (which now form the western territories of present-day Slovakia), Amadeus Aba controlled the northeastern lands, the Kőszegis ruled Transdanubia, James Borsa dominated Tiszántúl, and Ladislaus Kán governed Transylvania.
Ugrin Csák was the de facto ruler of Upper Syrmia.
He was styled as ispán of Syrmia, Valkó, Bács and Požega counties in August 1303, thus he was considered the omnipotent lord in ther region.
Historian Pál Engel argued he held these dignities since the beginning of the 14th century until his death.

Ugrin was the most ardent domestic supporter of Charles' claim to the Hungarian throne for a decade until his death.
The so-called Chronicon Posoniense ("Chronicle of Pressburg"; present-day Bratislava, Slovakia) also emphasized his importance several times, which fact suggests that the chronicle was written in the court of the Csáks' Újlak branch.
In the lists of barons, which were part of the royal charters issued by Charles in the first decade of the 14th century, Ugrin's name was constantly placed to the first place, ahead of the king's another mainstay Amadeus Aba.
Following Andrew's death, Ugrin had an important role in the subsequent events; under his guidance, Charles hurried Székesfehérvár, then Esztergom, where he was crowned king irregularly in the spring of 1301.
Being Pope Boniface's candidate for the Hungarian throne, Charles had always been unpopular, because the Hungarian lords feared that they would "lose their freedom by accepting a king appointed by the Church", as the Illuminated Chronicle narrates.
The majority of the lords supported Wenceslaus instead of him.
After Wenceslaus's coronation, Charles withdrew to Ugrin Csák's domains in the southern regions of the kingdom.

Despite his advanced age, Ugrin participated in the unsuccessful siege of Buda in September 1302.
Through his diplomat Benedict, he also played a decisive role in the concluding alliance between Charles and his cousin Rudolph III of Austria, in Pressburg on 24 August 1304.
According to the Chronicon Posoniense, Ladislaus Kán handed over his prisoner, the self
Andrew III made his maternal uncle, Albertino Morosini, Duke of Slavonia, in July 1299, stirring up the Slavonian and Croatian noblemen to revolt. A powerful Croatian baron, Paul Šubić, sent his brother, George, to Italy in early 1300 to convince Charles II of Naples to send his grandson to Hungary to claim the throne in person. The king accepted the proposal and the twelve-year-old Charles arrived to Hungary with his escort. They landed at Split in Dalmatia in August 1300. From Split, Paul Šubić escorted him to Zagreb, where Ugrin Csák swore loyalty to Charles, who also handed over Požega Castle to him. Ugrin placed the young pretender under his guardianship. Other sources confirm that he already took an oath of allegiance to the pretender prior to that. According to the Illuminated Chronicle, Ugrin was one of those lords, who requested Pope Boniface VIII to provide a new king for Hungary. In February 1300, Charles II listed Ugrin's name among those Hungarian barons, who urged him to send his grandson to Hungary. His familiaris and skilled diplomat, Benedict also visited the Neapolitan court several times in that period. It is possible that Ugrin turned against Andrew already in 1298, as he did not countersign the decrees of the national diet in that year, which aimed to destroy forts built without permission and ordered the punishment of those who had seized landed property with force. Ugrin did present neither before the king's judicial court in 1300, during a lawsuit regarding his former palace in Esztergom.
Andrew III died on 14 January 1301. With his death, the House of Árpád became extinct. A civil war between various claimants to the throne – Charles of Anjou, Wenceslaus of Bohemia, and Otto of Bavaria – followed Andrew's death and lasted for seven years. Hungary had disintegrated into about a dozen independent provinces, each ruled by a powerful lord, or oligarch. Among them, Ugrin's distant relative, Matthew III Csák dominated the northwestern parts of Hungary (which now form the western territories of present-day Slovakia), Amadeus Aba controlled the northeastern lands, the Kőszegis ruled Transdanubia, James Borsa dominated Tiszántúl, and Ladislaus Kán governed Transylvania. Ugrin Csák was the de facto ruler of Upper Syrmia. He was styled as ispán of Syrmia, Valkó, Bács and Požega counties in August 1303, thus he was considered the omnipotent lord in ther region. Historian Pál Engel argued he held these dignities since the beginning of the 14th century until his death.
Ugrin was the most ardent domestic supporter of Charles' claim to the Hungarian throne for a decade until his death. The so-called Chronicon Posoniense ("Chronicle of Pressburg"; present-day Bratislava, Slovakia) also emphasized his importance several times, which fact suggests that the chronicle was written in the court of the Csáks' Újlak branch. In the lists of barons, which were part of the royal charters issued by Charles in the first decade of the 14th century, Ugrin's name was constantly placed to the first place, ahead of the king's another mainstay Amadeus Aba. Following Andrew's death, Ugrin had an important role in the subsequent events; under his guidance, Charles hurried Székesfehérvár, then Esztergom, where he was crowned king irregularly in the spring of 1301. Being Pope Boniface's candidate for the Hungarian throne, Charles had always been unpopular, because the Hungarian lords feared that they would "lose their freedom by accepting a king appointed by the Church", as the Illuminated Chronicle narrates. The majority of the lords supported Wenceslaus instead of him. After Wenceslaus's coronation, Charles withdrew to Ugrin Csák's domains in the southern regions of the kingdom.
Despite his advanced age, Ugrin participated in the unsuccessful siege of Buda in September 1302. Through his diplomat Benedict, he also played a decisive role in the concluding alliance between Charles and his cousin Rudolph III of Austria, in Pressburg on 24 August 1304. According to the Chronicon Posoniense, Ladislaus Kán handed over his prisoner, the self
Ugrin Csák