So Arca here. I have many questions about Mega-Evolution today. Yup. So say we have Gyarados up against Empoleon. Stealth Rock is already active. Now, Gyarados mega-evolves. And then, Empoleon uses Roar. What would happen at this moment ? Gyarados would get out of the field ? Can my opponent mega-evolve another time ?
Once you Mega Evolved, you remain in that form until the match ends, regardless if you switch or get roared out. Once Mega Evolved, you are unable to revert back into its normal form, and the Pokemon will take hazard damage based on its Mega Evos typing.