Can I run my own database behind my FusionAuth Cloud instance?

[matth]

Short answer:

1. No. FusionAuth does not allow customers to access FusionAuth Cloud.

What you can do instead:

1. You can self host and run FusionAuth right next to your existing database.

2. You can create user attributes that tie FusionAuth users in the cloud to the users in your existing database.

For example, you could create a user.data.profile FusionAuth user profile object with the id of the user in the existing database. And then add a column on the existing side to store the FusionAuth user id. Then use webhooks to keep any shared attributes in sync.
https://fusionauth.io/docs/v1/tech/events-webhooks/

3. You can import everything to FusionAuth, potentially using a Connector.
   https://fusionauth.io/docs/v1/tech/connectors/

In all cases you'll need to migrate the auth related portions of the database to FusionAuth.

Here's a guide for that: https://fusionauth.io/docs/v1/tech/migration-guide/general

One final thought is you could wrap FusionAuth and your legacy database into a single API (that you write; possibly set up a proxy) so even if there are two databases/applications containing user data, it will look like one to all consumers of it.