Matrix, which is pretty much an encrypted and open-source Discord clone (at least in the same fashion as Lemmy would be a Reddit clone). I personally use Element to interact with it and have a matrix.org account, but Matrix is just like the fediverse, you can choose any instance or client you want, or even host an instance yourself. In your Lemmy settings you can set up your Matrix user, right below your email address as of 0.18.1, and if you do, a new buttons saying “send secure message” will show up on your profile, next to “send message”, which will redirect people trying to message you to Matrix.