Which episode did Daenerys Targaryen lose her dragon, Viserion, to a White Walker's spear?
The Night King, the leader of the White Walkers, threw a spear at Viserion, which killed him.
Instruction: Which episode did Daenerys Targaryen lose her dragon, Viserion, to a White Walker's spear?
Context: 
Response: The Night King, the leader of the White Walkers, threw a spear at Viserion, which killed him.