In the penultimate episode of season seven, Daenerys uses her trio of dragons to rescue Jon Snow and his cohorts from a White Walker attack beyond the Wall. Viserion aids his brothers in battle by breathing fire over the scene, but the Night King arrives and hurls an ice spear at the powerful beast. The blade fatally wounds the dragon who crash lands into a frozen lake.
The wights retrieve Viserion's corpse and the Night King revives the animal with his ancient magic. This new dragon's eyes glow blue and his scales appear forever frosted.
With Viserion as his mount, the Night King leads his army to Eastwatch where the demon beast blasts the Wall with blue fire, allowing the Undead Army to enter the Seven Kingdoms.