Baron Benincasa was the baron of the Misty Mountains following the defeat of the goblins at the hands of Bilbo Baggins, Thorin and Company and Gandalf. After Smaug was defeated and Thror's treasure was distributed, the Baron made his home, along with his personal army, in the Misty Mountains. During the War of the Ring, he led his army against Sauron and his army but they were ultimately killed. The Baron himself was slain by Orcs at the Battle of Helm's Deep while coming to the assistance King Theoden.
Becoming the Baron
After Thorin and Company cleared the Misty Mountains of many of the Orcs, the Baron and his personal army took over the Mountains, clearing out the rest of the Orcs and establishing a town. For 60 years, Benincasa and his people avoided contact with the outside world, except for Gandalf the Grey.
War of the Ring
At the start of the War of the Ring, Benincasa was hesitant to join the fight because his people were never harmed by Mordor and had no quarrel with Sauron or Saruman. However, he soon changed his mind after the deaths of several of his soldiers at the hands of the Uruk-hai, under the command of Saruman. He came to King Theoden's aid at the Battle of Helm's Deep but he and his men were overwhelmed because there was so few of them.
Death
After the Baron's horse was killed, he was forced to fight on foot. He slayed many Orcs but, due to his old age, was unable to fight them off forever. He was slain after several Orcs charged him.