Warning: Contains mild SPOILERS for House of the Dragon season 1, episode 2.House of the Dragon season 1, episode 2 sets up Vhagar, the dragon in the show most capable of rivaling the size and power of Balerion the Black Dread. The sheer amount of dragons in House of the Dragon is part of its core appeal. Where Game of Thrones only had Daenerys Targaryen's three dragons, House of the Dragon will have around 17 in total (though not all will appear in season 1), two of which have already been seen to great effect in the form of Syrax and Caraxes the Blood Wyrm.
Still, when it comes to dragons in Westeros, there's one who beats them all: Balerion. The Black Dread is the stuff of legend in Game of Thrones, where his massive skull can be seen down in the Red Keep's cellars. The dragon of Aegon the Conqueror himself, he was and remains the biggest dragon the Seven Kingdoms has ever seen, but unfortunately is no longer alive by House of the Dragon's timeline, having died around 94 AC (with his last dragonrider none other than Viserys).
Balerion's skull is once again seen in House of the Dragon but, unless there are flashbacks (which would be surprising, given there's no clear need for them) then Balerion won't be seen alive during the show. That's a shame, but one dragon will come close: Vhagar. The dragon is mentioned by Viserys during his talk with Laena Velaryon (whom he thankfully does not marry), and is currently MIA, with reports of her being seen over the Narrow Sea. That nicely sets up Vhagar's appearance later in House of the Dragon season 1, and it should be quite the sight to behold: as Viserys notes, she is arguably "too big for our world," and, by the time of the Dance of the Dragons, is almost equal in size to Balerion at his peak (and far, far bigger than a dragon like Caraxes). So although viewers won't get to see the Dread, Vhagar is going to come very close to matching him.
What Role Vhagar Plays In House Of The Dragon's Story
Warning: SPOILERS for Fire & Blood from this point.It's no coincidence that Laena is the one who takes such an interest in Vhagar: as established in the books, she becomes the dragon's first rider in years, following in the footsteps of Aegon's sister-wife, Queen Visenya, and Prince Baelon (Viserys' father). Indeed, she's said to have become a dragonrider before the age of 12, which is how old she is revealed to be in the episode. Though House of the Dragon changes the timeline (several events are happening years later than in the books), it could mean Laena will bond with Vhagar sooner rather than later. That itself is minor compared to what's to come, though: after Laena's death, Vhagar will be claimed by Aemond Targaryen, the second son of King Viserys and his new wife, Alicent Hightower. With Aemond in control, Vhagar will become one of the most powerful and destructive forces in the Dance of the Dragons.
By the time of the Dance of the Dragons, Vhagar is around 180-years-old, by far the oldest, biggest, and fiercest dragon in Westeros and the only one to come close to Balerion. Her size, battle experience, and the sheer heat of her breath are unparalleled, and seeing her in action should surpass anything else in either House of the Dragon or Game of Thrones. Assuming that House of the Dragon season 1 ends with the Targaryen civil war just beginning, then Vhagar could display her full, astonishing might right at the very end, but there'll be more to come as she's a key player for Alicent and Aegon's "greens" faction against Rhaenyra's "blacks" in the Dance of the Dragons. If done right, then seeing Vhagar in action should provide the same kind of thrill and awe that Balerion himself would provide.