So, i've been searching for an 'evil' Mage, well, not exactly evil but by no means an upstanding citizen with common morale standards.
specifically somebody who poses a threat, large or small, to the population of tamriel, or holds some power within tamriel.

I've been looking through the imperial library but all i can seem to find is accounts of Mannimarco and Mankar Camaron, and although they fit the bill they are not entirely what im looking for.

Any suggestions on where to look will be greatly appreciated.

Jagar Tharn- former Imperial Battlemage, he trapped Uriel Septim in Oblivion and impersonated him for ten years before he was stopped in TES I.

Aye Jagar Tharn works, but unfortunately dead, im looking (hopefully) for a living person, its for a fanfic/rp in the making, set after the events of oblivion

There's probably a reason vague threats don't tend to abide all that long.

The King of Worms that comes(?) after the one in OB?

The sload are necromancers. I wouldnt put it past those pustulent bloat-bags to bring good old N'gasta back from the dead so that he can teach them his "Soul snare" spell.

Let's see the list of evil mages:

Mankar Camoran-deceased
Mannimarco-deceased (although there's always the chance that he will be able to return, given his power and mastery of necromancy)
Jagar Tharn-deceased
Relmyna Verenim-resides in Shivering Isles but has shown no megalomaniacal tendencies
N'Gasta-deceased

Dang, they got a high mortality rate.

The Telvanni are quite often amoral or even nasty, although they prefer to keep to themselves and fight amongst themselves..

Psijics (under discussion probably not evil).

Mistress Dratha, she is leaning from amoral to evil and she is a [censored].
How big threat she is for general population? I'm not sure.

Some rogue Telvanni mages can be easily labeled as evil, sometimes they pray on living people to secure material for their experiments (for example mage in Odirnirian tower).