SuperTokens is an open source authentication solution offering features like: Different types of login: Email / password, Passwordless (OTP or Magic link based).

SuperTokens.com

I’m working on a multi tenant app with a centralized user store. The idea is to give users a subdomain, `user.saas.com`, but also give them the option to use their own domain if they want, `user.xyz`.

My users will mostly create accounts via SSO and I don’t want to have to add every custom domain to my redirect URLs for say, Discord’s Oauth2. Can SuperTokens handle being that service?

You could make the users all redirect back to a common domain and then redirect them further to their respective sub domain

Any suggestions for how to pass the token to that other service? Wouldn't I basically be writing my own Oauth2 service at that point?

Yea. You would. We are not yet an OAuth provider, so you would have to pass back the tokens as a redirection.

yes you can. We have docs for implementing custom providers