Nihilus' Power
Well, I managed to dig up their huge quote Traya gives about what Nihilus does. It will thoroughly destroy the Revan knows his power argument.
I'll quote the whole thing but I'll bold and underline the parts you should know if you don't want to read it all.
Kreia: "He... if he can truly be called a man any longer... is one of the dark lords that pursues you. I do not think he knows what you are, not yet. He spared the Miraluka, and that may have been the last shred of feeling that exists within him. Keep his slave close to you. I suspect there was a reason he spared her... and perhaps a reason that she survived when the rest of her people and the Jedi did not. Perhaps he is bound to her... as I am bound to you. If so, there may be a death served by hers. It is a technique that is almost as old as the Sith themselves... it is a means of severing connections between life, the Force, and feeding upon the death it causes.It cannot be taught... it can only be gained through instinct, through experiencing its effects, first-hand.{Quietly}Yes. And he fed upon its destruction - it will sustain him, for a time.Because it is not something that can ever truly be controlled... and it leaves nothing to conquer in its wake.And it rules him, not the other way around. It has its own will, its own instincts.
{Chiding}Power? Do you think so?{Shakes head}You would be wrong. There is no strength in the hunger he possesses... and the will behind his power is a primal thing. And it devours him as he devours others - his mere presence kills all around him, slowly, feeding him. He is already dead, it is simply a question of how many he kills before he falls.Nothing is impossible with the Force. It is an energy that flows through all living things. And like energy, it may be harnessed, channeled, and consumed at times. It may even be a substance that can burn and ignite.Do not think of his power as one would a weapon, or one of your warships of the Republic. It is terrible, but it is still a subtle thing. The sect of assassins that chase you feed on the Force... what he does is simply the pinnacle of what they could achieve, in time. And that is why they - and their techniques - must be wiped out. No one again must experience and learn what her master did. As much as one may use the Force to bolster the wills and strengths of others, the reverse is possible, though not often used. Instead of sending one's will through connections in the Force, instead such connections are drawn upon, fed upon, and drained completely. Then you understand how terrible such a power is. And why it must be ended.It is an empty road to the dark side, and by traveling it, the price is death before one's time. "
As you can see, Traya says clearly that the immense power Nihilus has can not be taught. It is an empty road in the dark side and if Revan had the instinct and experienced it, he may be like Nihilus too. Which he isn't so how could he have learned it?
The power is clearly not a Force Drain, though. It requires using the Force Bonds explained in KOTOR II but then severing them and cutting off life itself before feeding on the death caused by this.
Also Kreia says Nihilus and his Assassin's technique must be killed so this power is lost forever. If you could learn it or if Revan had known it, she would have wanted him destroyed it.