The grind between Deino, Zweilous, and Hydreigon is the longest that Pokémon players will experience. Hydreigon is fast and is one of the better competitive Pokémon due to its high Special Attack. Tailwind and Draco Meteor are enough reasons to want a Hydreigon on its own, but evolving Zweilous takes a lot of investment. Using EXP Candy will speed up the process, making Raids pretty important to saving time.
What level does Zweilous evolve into Hydreigon in Pokémon Scarlet & Violet?
Deino evolves into Zweilous at level 50 and Zweilous evolves into Hydreigon at level 64. This makes it the highest-level evolution for any Pokémon in the series, next to Drakloak and Larvesta's evolutions. It is highly recommended to either farm Pokémon that award great amounts of experience, like Chansey and Blissey, or to do Tera Raids to speed up the process. A refresh on how to force Pokémon Outbreaks to work can help determine how to reliably get them to spawn.
Where to catch Zweilous in Pokémon Scarlet & Violet
Deino is exclusive to Pokémon Scarlet, but Hydreigon is often found in Tera Raids when playing online for Pokémon Violet players. North Province Area Two is the easiest place to find a Zweilous when searching through caves. Trading is also a great way to obtain the Pokémon, as is joining a friend's Union Circle and traveling to their spawn locations with them. Both Pokémon tend to show up near caves, and if players are having trouble finding Zweilous, the Deino located on the East side of the map will usually be around level 50. One Rare Candy will make them evolve and get one step closer to Hydreigon.
To know what Nature is best for these Pokémon, check out Best Nature for Deino, Zweilous, & Hydreigon in Pokémon Scarlet & Violet, and to know about which Dragons are the best, check out Best Dragon Types in Pokémon Scarlet and Violet on Pro Game Guides.