One of the interesting things about the Elixir/Phoenix community is that building your own authentication system seems to be encouraged. Even if you're using one of the plugs available (guardian?) you still have to do a ton of manual lifting.
It really depends what you are going for. There are libraries out there that are pluggable for lots of different auth mechanisms like https://github.com/ueberauth/ueberauth
If you want something like Devise from the Rails world, there is also Coherence. https://github.com/smpallen99/coherence