Guide
The Plaguerot Tauralus is purchased from Su Zettai but requires you to have obtained at least 39 renown before it will become unlocked.
Renown is earned weekly and it will take around 17 weeks to obtain enough Renown (although catch up mechanics makes this much faster). Once you have enough you can purchase the mount for Anima.
This mount is locked to the covenant Necrolord, even once earned you will not be able to use this on any other covenant.