There are some “leaked sources” of Ragnarok online and Mu Online out there, which people use to create private servers of those games. Technically you’re not allowed to do it but as long as you only do this to your friends (and don’t make money out of it) you should be fine.
Another option is not an MMO, but you could host a Valheim server (survival game with Viking theme) and put a password so only your friends can enter.
Other than those options, I personally don’t know any open source mmorpg you could host like that.
There are some “leaked sources” of Ragnarok online and Mu Online out there, which people use to create private servers of those games. Technically you’re not allowed to do it but as long as you only do this to your friends (and don’t make money out of it) you should be fine.
Another option is not an MMO, but you could host a Valheim server (survival game with Viking theme) and put a password so only your friends can enter.
Other than those options, I personally don’t know any open source mmorpg you could host like that.