After the famous council of Ottoman vassals at Serres, in 1395, Theodore, who was one of the princes summoned by Bayezid to Serres, was compelled to sign the cession of Argos and Monembasia to the Osmanlis. He was then thrown into prison, and Bayezid contemplated having him assassinated. But before the cities could be delivered to the Ottoman emissaries, Theodore escaped, and declared the cession null and void.[561] The first impulse of Bayezid was to send an army upon the heels of Theodore. This punitive expedition was postponed on account of the activity of Sigismund, and the necessity of defending the northern frontiers against the Hungarians.[562] In the spring of 1397, while Bayezid was superintending the construction of a mosque at Karaferia in Macedonia, he received a visit from the Greek bishop of Salona, who laid before him a formal accusation of adultery, sorcery, and oppression against Helena Cantacuzenos, who had been ruling the Duchy of Salona with her paramour after the death of her husband, Louis Fadrique. The bishop invited Bayezid to enter Greece, depicting to him the wonderful hunting he would have in a country full of game.[563] The promise of good sport with the falcon was not needed. It had long been Bayezid’s intention to extend his sovereignty into the Greek peninsula. He had against Theodore not only the old count from Serres, but also the complicity of the Morean despot in the Nicopolis crusade. At the head of his army, he set out upon the first Ottoman invasion of Greece. In Thessaly, Larissa, Pharsala, and other strongholds surrendered without striking a blow. For thirty years the Greeks of Thessaly had felt that the Ottoman conquest was inevitable. When Bayezid crossed the pass of Thermopylae without opposition, Helena hurried to meet him. She offered her principality, her daughter, and herself to the conqueror. Bayezid did not want the duchess. She was set at liberty immediately. But the beautiful grand-daughter of John Cantacuzenos was sent to his harem. The duchy of Salona, in which was the shrine of Apollo, with all of Phocis, Doris, and Locris, was added to Thessaly, and made an Ottoman province.[564] Bayezid by this time had tired of the campaign. He felt an irresistible call to return to the pleasures of the court. His military interests were beginning more and more to be centred upon an extension of his power in Asia Minor--the policy that was soon to prove his undoing. But there remained Theodore and the Morea to be dealt with. He left Yakub and Evrenos, with an army of fifty thousand, in charge of the invasion of the Peloponnesus.