Originally posted by katsuragi:

I never did find a reveal stone but instead got lucky and came across a small plains island with the boss altar on it, clearly visible from the sea

Yeah, Yagluth altar is pretty prominent feature in the landscape. I guess it's all intended, it used to be the final boss of the game for a good while after all.

It's also not a bad thing of having him little hard to find, since nowadays defeating him triggers seeker events and those are bad news for all but the most prepared players.