Viserion

Viserion is a recurring character in the A Song of Ice and Fire series, as well as its television adaptation, Game of Thrones. Viserion is one of Daenerys Targaryen's three dragons, or "children", but after being killed and subsequently resurrected as an undead ice dragon by the Night King during Season 7 of the television series, has become a villain as well as the Night King's personal mount. Viserion is responsible for the partial destruction of the Wall at the end of Season 7. This has yet to happen in the novels, as the show is currently ahead of the books in the continuity.

Season 7
When Daenerys learns that Jon Snow and his group of adventurers were struggling against the White Walkers and the undead army beyond the Wall, she and her three dragons fly to rescue them in a desperate mission. During the mission, the Night King throws a spear made of ice through Viserion's neck, killing him instantly and causing him to fall through the sky into the frozen lake. After Jon, Dany and the others flee, the wights use several massive chains to drag Viserion's corpse from the frozen waters. The Night King then uses his magical powers to revive Viserion as a massive wight dragon.

Later, when the army of the dead finally arrives at the Wall, the Night King is shown riding Viserion. Having apparently gotten more powerful now that he had been raised into undeath, Viserion breaths blue fire and destroys a portion of the Wall, allowing the White Walkers and their undead minions to finally cross over into Westeros for the first time in thousands of years.