With Capricorns being so disciplined and hard-working, they would make an ideal apprentice for Darth Plagueis, particularly on his tireless quest to achieve immortality. Interestingly, though ...

We also see Plagueis as the apprentice of Darth Tenebrous, a Bith dark lord who in the first chapter takes his Large Adult Student to the planet of Bal’demnic to find a cortosis vein.