Since you know the account it was Migrated to and you know it's not your account and is someone else's, then there's sadly very little you can do to regain your progress. - Unfortunately, once a Blizzard Battle.net ID has been migrated to a Steam account, neither of those accounts can be used for another Migration. Sadly, if your Blizzard Battle.net account was Migrated to a Steam account that you don't own, then you've lost access to that data and those licenses. It cannot be undone or exchanged to a different Steam ID. You can work with Steam to report the theft of your data, and you can work with Steam to try and convince them to give you the licenses the other Steam ID has, but Bungie will not move the character save data from one Steam ID to another. The only way you'd be able to regain your progress would be to convince Steam that you're the owner of that Steam ID so you can take control of it, and you have to hope that it's not been banned for doing something naughty in Destiny 2. Your Blizzard Battle.net ID logged in and set up Migration to a Steam ID. The only way to set it up was to log in with the relevant accounts. Bungie doesn't have their own login system and relies on the authentication methods of those other parties. On Bungie's side of things, your account set up the Migration and the Migration was successful.