Here's what it does at level-16:

You activate and get an instant 600HP. It can be used to bait people like a steroid-red potion.

Nasus goes into his ultimate-form and begins draining 5% of people's health in magic damage. This is mitigated by magic resist. It will take 5% of the enemy HP and calculate it against the MR; if it's over 240 then it caps at 240. It's unlikely for this to ever happen against an enemy champion. The main reason for the cap is to prevent someone from killing Baron a little too easily.

What damage is being done to a person is then put back into Nasus's attack damage. This is why if you're completely surrounded by enemies and you hit some weak sod in the face with a Siphon Strike you notice a rather large spike in damage.

The range of his ult -- that is the area of effect -- is something I still don't know, but I'm under the impression that it may be about twice the size of Alistar's pulverize range. Not too small, but not exactly long-range either.

Your enemies drift away, blown into eternity like the sands of the desert.

It doesn't return all of the magic damage you deal as an attack damage bonus. The % converted is small, like .03% or something. Try it on the dragon. I'll see if I can find the actual %.

Apparently I'm wrong. This is what you're looking for.

https://leagueoflegends.com/board/sh....php?p=3608026

Confirmed by Guinsoo.

I have no idea of the black magic working into Nasus' Ultimate's damage formula, but to test the range just go into a practice game, buy a rylai's and see how far away minions start freezing. That's how I tested Amumu's despair range a couple months ago.

I would love to see the bonus damage that is granted. Then again, if it is shown, a lot more people will probably start playing Nasus due to his spike of bonus damage and I'll be out of a job

So.....according to guinsoo...

5% of the raw damage you deal, is added cumulatively to your bonus damage during his ultimate.