To support a ruler of this kingdom defending a siege, Calixtus III began the practice of the ringing of the Noon Bell. A ruler of this (15[1])kingdom used the ouroboros and crusader imagery when founding the Order of the Dragon. After receiving other titles, that ruler of this kingdom organized the election of Pope Martin V at the Council of Constance. A ruler of this kingdom hired the standing (*) “Black Army” of (10[3])mercenaries and amassed the largest library outside of Italy, which was later brought to Istanbul after his 1490 death. Matthias Corvinus and John Hunyadi ruled this kingdom, which fought the Ottomans at battles like Varna before being decisively defeated in 1526 at Mohács [moe-hatch]. For 10 points, name this kingdom founded by the Árpád dynasty that became the main eastern part of the Habsburg realm. ■END■