It is a simple process I saw when Deino Evolves to Zweilous and Hydreigon. This involves leveling in pokemon scarlet.you know In a particular time Deino will become Zwellous as it reaches level of 50 and Hydreigon after attaining level 64.you can capture Deino in the scariest version of the game. Although, you're playing pokemon violet. Owners will have to find a friend to trade with in order to obtain Deino.