He was an old, powerful large male. Of all the dragons in Westeros, only Vhagar was older and larger. Vermithor was thrice the size of Prince Daeron Targaryen's she-dragon, Tessarion. He was bronze in color with great tan wings.
He was almost a hundred years old at the time of the Dance of the Dragons. Due to his size he was not as nimble in the air as some of the smaller, younger dragons. His flame in the fullness of his power melted steel and stone. Unlike his wild brethren, Vermithor was accustomed to men and tolerant of their presence, but when riled he was a truly terrifying dragon, and rained down hell on the armies he fought during the Dance. It is noted by maesters that, during his final battle with Tessarion and Seasmoke, the latter was no match for him at all, and Vermithor would have effortlessly won the battle if it were just those two.
During the civil war, when it was decided by Jacaerys Velaryon that the Blacks needed more dragonriders, so many men came forward to answer the Prince’s call. This proved to be very dangerous, Lord Gormon Massey was burned to death while approaching Vermithor. The dragon eventualy bent his neck to a blacksmith’s bastard called Hugh Hammer and was ridden by him during the civil war.
Vermithor took part in and survived the Battle in the Gullet. Along with Silverwing, Sheepstealer, Vermax and Seasmoke, Vermithor descended upon the ninety warships sailing under the banners of the Three Daughters.
Along with Caraxes, Silverwing and Sheepstealer, Vermithor made a lair in the Dragonpit before flying off to battle. At Tumbleton, the dragon was kept outside the town, unchained in fields to the south along with Silverwing. Silverwing and Vermithor oft coiled about one another in the fields. During the Treasons of Tumbleton Vermithor and Silverwing were used by to the Two Betrayers to scourge the town with whips of flame from one end to the other.
During the Second Battle of Tumbleton the riderless Vermithor rose into the air and was attacked from above by the only dragon with a rider: Seasmoke. Vermithor was no more than twenty feet above the battle when Seasmoke slammed into him from above, driving him shrieking into the mud.
The riderless Blue Queen soon joined the fray and all three fought to the death on the ground amidst mud and blood and smoke. Vermithor killed Seasmoke when he locked his teeth into his neck and ripped his head off. He attempted to take flight with his prize still in his jaws but his tattered wings could not lift his weight. After a moment he collapsed and died.
It is said that after dark Silverwing descended to land beside her slain cousins. Later, the singers would tell of how she tried to lift Vermithor’s wing three times with her nose, as if to try and make him fly again, in vain.