I want to create a plugin that will override the login before the admin page.

I want the user to be redirected to the CAS server and then return back.
I am not quite sure how to do it.
Any headers for the events that I need to override and what way do you advise me to take?
What things should I look out for?
I have tried checking other login plugins but either I couldn't make them work or they were older versions.
Any help will be appreciated!!
Thanks in advance

Update

Included with composer jasig phpCAS package inside my plugin
used it my plugin.php using namespace \phpCAS::

Override events used:

1. 'onUserLoginAuthenticate' (before login form)
2. onUserLogout'

Currently my plugin works in parallel with grav's login plugin and I haven't figured out the best way to override the login plugin.
In short I want to login as admin through only my own CAS login plugin.
I tried disabling login but grav's admin plugin needs it enabled.
Is there an advisable way to do this?