In the Middle Ages, the island of Euboea and its capital Chalkis were known to the Latins under the name "Ne(i)groponte". According to the Partitio terrarum imperii Romanie (treaty for the partition of the territories of the Byzantine Empire), the central part of Euboea was awarded to the Latin king of Thessalonika Boniface of Montferrat (1204-1207). Boniface conquered the island and ceded it as a fief to the Flemish noble Jacques d’ Avesnes (1204), who built the castle of Chalkis and garrisoned it. After the death of Jacques (before August 1205), Boniface ceded the island to three barons from Verona, Italy: Ravano dalle Carceri, Giberto dalle Carceri and Pecoraro da Mercanuovo.
The three barons were given the name triarchs (terzieri or terciers), since each one of them got one third of the island, namely one of the three baronies into which Euboea was divided. Those were the northern section with Oreos as its capital (terzero del Rio), the central section with Chalkis as its capital (terzero della Clissura, Terzerius a clesura ultra), which was common for all three barons, and the southern section with Karystos as its capital (terzero di Caristo). Towards the end of 1208, Ravano dalle Carceri became the lordof the entire island. They were now called sires de Nigrepont and a short while later, in 1210 and 1212, dominus Nigropontis or dominus insulae Nigropontis.
Ravano ranged himself with the Lombard lords of Thessalonika in his struggle against the Latin emperor of Constantinople (1207-1209). In order to deal with the political isolation of his island caused by his personal involvement in the rebellious movement of the lords of Thessalonika, he contracted in March 1209 a treaty of alliance with Venice, by which he acknowledged the Venetian dominion in Euboea. Through the treaty of March 1209, Venice acquired significant commercial privileges in the island. A while later the first bailus was appointed. He governed the communities that his compatriots held in the island, as the representative of the doge. On 20 May 1209, Ravano also acknowledged the dominance of the Latin emperor of Constantinople and became his liege. Later, possibly in 1248, the dominion of the Latin Empire of Constantinople in Euboea was transferred to the prince of Achaia (1246-1278). However, the dependence of the island on the prince of Achaia remained theoretical, as well as the prior dependence of the island on the Latin emperor of Constantinople.
After the death of Ravano dalle Carceri in 1216, the then Venetian bailus Pietro Barbo intervened forcefully in the feud of the six relatives of Ravano, who claimed his inheritance dividing the island into six districts (sestieri). The Venetian bailus gradually became the real governor of the whole island. Euboea acknowledged its total submission to the Venetians, when at the margins of the feud between its triarchs and the prince of Achaia William II de Villehardouin, the triarchs contracted, on 14 June 1256, an agreement with the Venetians, by which they acknowledged Venice as their sole ruler.
William had intervened as ruler of the triarchs of Euboea claiming the hereditary transfer of the half terzero of Oreos, which belonged to his wife Carintana dalle Carceri, who had died in the meanwhile. Between William and the triarchs of Euboea a strong feud broke out, which culminated in the battle of Karydi in 1258, where the prince of Achaia prevailed over his enemies. The conflict was finally settled through the treaty of 1262.
Directly associated with the claim of the half terzero of Oreos is the bronze obol that bears the inscription G(ULIELMUS) P(RINCEPS) AC(HAIE) (William Prince of Achaia) + NEGRIP(ONTE) (of Chalkis). The coin dates from the first period of the feud between the prince of Achaia and the triarchs of Euboea (1255-1256). On the reverse of the coin the Latin number III has been inscribed, which is believed to signify the status of William as triarch of Euboea.
The contribution of the Venetian baili to the defence of the island had been significant in periods of external enemy attacks, although Venice had not always been in the position to offer important military assistance to the triarchs. Some of the attacks were that of the prince of Achaia in 1258, the operations of the knight of Karystos Licario, during which (1264-1280) he conquered forts of the island with the support of the Byzantines from 1271 onwards, and the Turkish naval attacks of the 14th century. The Lombard sestieri and the Greek nobles took an oath of fidelity to the Venetian bailus, thus acknowledging Venice as the prevailing power on the island. The Lombard sestieri had their capital in Chalkis, which was the seat of the Lombard judge (podesta).
Towards the end of 1317 and beginning of 1318, Karystos was conquered by the Catalan Don Alfonso Fadrique, vicar-general of the duchy of Athens and illegitimate son of Frederic II, king of Sicily (1295-1337). In 1319, the Catalans entered into a treaty with the Venetians, which was renewed in 1321, according to which Don Alfonso maintained Karystos under his jurisdiction. The official dominion of Venice spread gradually throughout Euboea. In 1342, Venice bought the fort of Larmena and Karystos in 1365. Around 1350, during the third war between the Venetians and the Genoeses, Chalkis suffered an attack by the Genoeses, who plundered and burnt down the city. In 1390, when the heir of the last triarch of Euboea, George III Ghisi, died, the island came entirely under Venice.