No stones makes Zweilous evolve. It is really anoying, but you wil have to train it to level 64 if you want a Hydreigon. Zweilous is the Pokemon who needs moost training of all to evolve.
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Zweilous evolves at level 64
zweilous evolves into hydreigon at level 64.
Deino evolves into Zweilous at level 50 which is the highest level that the first Pokemon of a 3-stage evolution evolves. Deino doesn't evolve with a stone.
Lv64
Train it to level 54 and it will evolve into Zweilous