Nihilus’ exact identity is never specified in K2 - I had a specific origin in mind, but not a name, if that makes sense, and what I say hear is not canon: He was created when Malachor V was torn apart, and he was intended as the Exile’s other half, one that took a more self-destructive path rather than denying the Force during that battle that ended the Mandalorian Wars.
As much as Nihilus embraced the Dark Side’s Force talent of consuming force sensitives (and other life, such as the mass Mandalorian slaughter), the Exile took the higher ground and cut himself off from the pull of such power. As powerful as Nihilus’ ability is in the short term, the drawback is that it robs the user of almost identity but hunger, which is why is never employed by the Sith Lords of old... who had no wish to sublimate their identities for any reason.