The details of the battle, including its precise date and location, are disputed as the primary sources give contradictory information; modern scholars usually place it either in July or in September, somewhere in the plain of Pelagonia or near Kastoria.

It appears that the barely concealed rivalries between the Epirote Greeks and their Latin allies came to the fore in the lead-up to the battle, possibly fanned by Palaiologos' agents.

As a result, the Epirotes abandoned the Latins on the eve of the battle, while Michael II's bastard son John Doukas defected to the Nicaean camp.

Like the earlier Norman kings of Sicily, Manfred had his own ambitions in the Balkans, including Constantinople itself; and the enforced residence of his half-sister, Constance, the widow of Vatatzes, at the Nicaean court, only aggravated his feelings towards Nicaea.

Already in the autumn of 1258, his army crossed over into Europe, under his brother John Palaiologos, who held the quasi-imperial rank of sebastokrator, and the megas domestikos (commander-in-chief) Alexios Strategopoulos, and wintered in Macedonia, where it was joined by local levies.

Michael II of Epirus, who was encamped at Kastoria, was caught off guard by the rapidity of their advance, and when the Nicaeans crossed the pass of Vodena to face him, he was forced to hastily retreat with his troops across the Pindus mountains to the vicinity of Avlona, held by his ally Manfred.

In their retreat, which continued even during night, the Epirotes reportedly lost many men in the dangerous mountain passes, while the Nicaean generals captured Ohrid and Deavolis and other cities.

Manfred, preoccupied with his conflicts against the Guelphs in central Italy, did not come in person–although his presence is erroneously reported by near-contemporary sources like Nikephoros Gregoras and Matteo Spinelli—but sent 400 superbly outfitted German knights, who probably landed at Avlona to join Michael II's forces.

The duke at the time was Ulrich III, but he ruled for many years after 1259, and was probably not at the battle, where the Chronicle maintains that he was killed at the hand of Geoffrey of Briel, the Baron of Karytaina.

The modern editor of the Greek version of the chronicle, Petros P. Kalonaros [el], opined that the "Duke of Karantana" is a fictitious character symbolizing a brave warrior, and the name was chosen possibly under the influence of a corruption of the name Karytaina.

[27] However, according to the historian Deno John Geanakoplos, "the statements of the Chronicle of the Morea are often exaggerated", and "one gets a clear impression from the sources [...] that the allied forces surpassed those of Nicaea in size".

[35] Gregoras and Pachymeres, while following the earlier account of Akropolites in the main, contain elements also found in the Chronicle, including the role of a Nicaean agent, and praise for the valiant conduct of the Achaean nobles.

[38][39] The exact location of the battle has been disputed, as the only clear toponym given in the sources is Boril's Wood (Βορίλλα λόγγος), which has been variously placed by modern researchers close to Prilep, Kastoria, or Bitola (then known as Pelagonia).

Faced with this constant harassment, Akropolites reports that the morale of the Epirote army withered, and Michael II with his troops withdrew towards Prilep, while John Doukas deserted the allied cause and went over to the Nicaeans.

[44] Pachymeres offers a completely different version, highlighting the discord present among the allies even before they met with the Nicaean army, allegedly as the result of some Achaean knights coveting John Doukas' beautiful Vlach wife.

The first he met and to whom he dealt a blow of the lance was he who was called Duke of Carinthia, and striking him on the chest, where his shield was raised for protection, he flung him lifeless onto the ground together with his horse.

The lance which he held shattered into three pieces, and so he quickly drew his sword and began to do battle in earnest with the Germans, and all those who came to fight him he mowed down like hay in a field.

[5][59] Already in 1260, Michael Palaiologos attacked Constantinople, as one of the knights taken prisoner in Pelagonia, and whose house was in the city walls, had promised to open a gate for the emperor's troops.

Strategopoulos and Petraliphas crossed the Pindus Mountains, bypassed Ioannina, which they left under siege, and captured the Epirote capital, Arta, forcing Michael II to flee to the island of Cephalonia.

[64][65] In the next year, however, the Nicaean successes were largely undone: John Doukas defected back to his father, and Michael II with an Italian mercenary army landed at Arta, and the Epirote population rallied to his cause.

This was ratified by the so-called "Parliament of Ladies" (as most of the male nobles of Achaea were prisoners), and in early 1262 Villehardouin was released, and the forts of Monemvasia and Mystras, as well as the district of Mani, were handed over to the Byzantines.

No longer able to confront the resurgent Byzantines, Prince William turned to the successors of Manfred of Sicily, the Angevins of Naples, for aid,[76] as, faced with a common enemy, did the Greek rulers of Epirus and Thessaly.