Mechanics in VR?

I would like to know where/how I could acheive showing an overlay of where the horizon is in VR/realty?

I'll be using Unity/C# and either the Rift or Rift S.

What I want to be able to do is add a mechanic where I can see a line/outline of where the real earth horizon is in VR. Maybe like some sort of overlay just off to the side of the screen?

Any ideas?

To be clear my Skill set with VR is OK and Unity is Adavanced (Been using unity since 2013). Just can't figure out what to tap into to be able to display an overlay of the horizon. Brainfart moment.