Neither. It uses all imbuements on your ultimate, that's it. No requirements.
Though if you don't have any extra imbuement damage it won't be that strong. The long cooldown isn't great either.
Not sure if a single of the imbuements gets stronger if you have points in it, but it's probable.
Anyway, I've found that amulet half a dozen times, it's the unique that drops the most for rogue. You'll probably get one soon.