|The title of this article is conjecture based on information revealed in the A Song of Ice and Fire novels or related material and may be subject to change.
Vermithor was one of the dragons that belonged to House Targaryen. It was once the mount of King Jaehaerys I Targaryen. During the great civil war known as the Dance of the Dragons, Vermithor was the mount of Hugh Hammer.
Vermithor was left riderless on Dragonstone island for many years after the death of King Jaehaerys I. During the Dance of the Dragons, Rhaenyra Targaryen's faction - the Blacks - was based on the island, but realized that they possessed more dragons than riders. It was believed that only those of the Targaryen bloodline could successfully bond with dragons, but over the generations the Targaryens had produced numerous bastard children and grandchildren with the commoners of the isle.
Rhaenyra put out a call for any who believed they were a bastard of Targaryen blood to attempt to mount and bond the six riderless dragons, offering reward of gold and title. Many of these bastards - called Dragonseeds - died in the attempt, but ultimately four of them succeeded. Vermithor was claimed by the bastard called Hugh Hammer.
Hugh and Vermithor fought on Rhaenyra's side in the civil war for some time, but during its later stages Hugh decided to betray her. When a large Green army of House Hightower advanced towards King's Landing after a long journey up from the south, Rhaenyra sent an army with two dragons to counter it: Vermithor ridden by Hugh Hammer, and Silverwing ridden by Ulf the White. In the ensuing Sack of Tumbleton, however, both Hugh and Ulf switched sides and turned their dragons against Rhaenyra's own army.
Luckily for Rhaenyra, rather than continue to advance on nearby King's Landing, Hugh and Ulf instead chose to drink and whore amid the ruins of Tumbleton with the Green army, unable to agree on what specific course of action to take next.
In the books
At the start of the Dance, Vermithor is stated to have been the largest living dragon after Vhagar herself, last of the first generation Targaryen dragons. Vermithor's scales were bronze with tan highlights, earning him the nickname "the Bronze fury".
Vermithor was famously the mount of King Jaehaerys I Targaryen, the Old King whose great and prosperous reign lasted 55 years. Jaehaerys I's sister-wife Queen Alysanne Targaryen was also a dragon-rider, and her mount was Silverwing. It has been speculated that there is some sort of magical/psychic or at least emotional bond between dragons and their riders, so that they like friends of their rider and scorn their enemies. In this case, Vermithor and Silverwing were actually a mated pair, mirroring the marriage between their two riders, and continued to remain so even after their original riders died. They "oft coiled about each other in the fields" when their armies weren't on the move during the Dance. Their matings may have produced some of the younger dragons at the time of the Dance.
At one point Jaehaerys I rode Vermithor on a royal visit to Winterfell, along with Queen Alysanne riding Silverwing, and four other as-yet-unidentified dragons. On a whim, Alysanne rode further up to The Wall itself, and Jaehaerys I followed on Vermithor.
The novels never give the lineage of any of the Targaryen dragons, though it is fairly clear that Vermithor was one of the powerful second-generation dragons hatched after the Targaryen Conquest. Vermithor was nearly 100 years old during the Dance, putting his hatching somewhere around the years when Aegon I died.
Live-readings that George R.R. Martin has done of his more extensive private notes mention that towards the end of Aegon I's life, there were about half a dozen dragon hatchlings. When Aegon I died several localized rebellions sprang up against his weak older son Aenys Targaryen, but Maegor helped put them down riding Balerion. Following this, two new dragons hatched at the same time, which was taken as a good omen for relations between the two brothers in the future (which turned out to be false). These two hatchlings may have been Vermithor and Silverwing, given how they were always a pair (Vermithor ultimately grew larger than the previous half-dozen, such as Dreamfyre, but this may just be up to individual variation). This also matches the dates given: if Vermithor was one of the earlier half-dozen dragons he would have been slightly over 100 years old during the Dance.
Vermithor was left riderless after Jaehaerys I's death, and had no other until he was claimed by Hugh Hammer 30 years later during the Dance. Of the six riderless dragons left on Dragonstone island, he was one of the three who were only semi-wild; compared to the other three who had grown to adulthood without riders at all. As a result, while Vermithor killed a few Dragonseeds who attempted to mount him, he was relatively accustomed to the presence of men and before too long accepted Hugh as his rider.
Hugh rode Vermithor into combat during the great Battle of the Gullet. The Season 5 Histories & Lore featurette recounts events in the wrong order, presenting it as if the loss of Vermax at the Gullet is what prompted Rhaenyra to look for dragonseeds to increase her number of war-capable dragons. In the prequel novellas, however, it was actually the loss of Meleys at the Siege of Rook's Rest which prompted this (the animated featurette greatly condensed this to only briefly mention the Battle of the Gullet in passing). Hugh then rode Vermithor during Rhaenyra's sudden seizure of King's Landing, though the defenseless city fell so quickly that he saw little if any combat.
About half a year later, Hugh and Ulf betrayed against Rhaenyra during the Treason of Tumbleton, turning Vermithor and Silverwing against the Black army and joining the main Green army of the Hightowers which had finally advanced up from the south.
After Vhagar died in the Battle Above the Gods Eye, Vermithor was left as the most powerful living dragon, for the last quarter of the two-year long civil war (about six months). For that matter, all of the other powerful second-generation dragons (Meleys, Caraxes, Syrax, etc.) were dead except for Silverwing, whose rider had partnered with Hugh. Even then, Vermithor is directly stated to have been the largest and most powerful dragon after Vhagar even at the beginning of the Dance. With Sunfyre also dying of his wounds on Dragonstone, only two other dragons remained, both younger and smaller (apparently third-generation) dragons: Seasmoke (possessed by the Blacks, fleeing to the Riverlands) and Tessarion (ridden by Aegon II's youngest brother Daeron). Seasmoke and Tessarion were of comparable size, both only around a third Vermithor's full size. Thus possession of Vermithor as his mount at this point gave Hugh the idea to proclaim himself king, glibly on the same grounds that Aegon I did during the original Targaryen Conquest: he had the world's largest dragon.
Vermithor ultimately died during the climactic Second Battle of Tumbleton, a chaotic battle-royale between the last living dragons and remnants of the Black and Green armies, grappling with each other amid mud and blood and smoke, the darkness of the night-time attack lit by their flames.
Vermithor was apparently named by George R.R. Martin as an homage to Vermithrax Perjorative, the dragon in the film Dragonslayer, which he has cited as one of his favorite movies.
Meleys · Meraxes · Morghul · Seasmoke · Sheepstealer · Shrykos · Silverwing · Sunfyre
Stormcloud · Syrax · Tyraxes · Valryon · Vermax · Vermithor · Vermithrax · Vhagar