Karkaroff is, or was, once a Death Eater, but after
Voldemort's downfall he turned informer to save his
own skin, and thus made himself an enemy in
Voldemort's eyes.

He became Headmaster of Durmstrang Institution under
circumstances which are not revealed, and proceeded to
sign the agreement for the Triwizard Tournament to be
held, under the circumstances named.

A man of a very excitable temper, (nervousness I
think), he explodes at anything, as was witnessed when
Krum was Stunned by Barty Crouch Jr., masquerading as
Mad-Eye Moody.

While ratting on the remaining Death Eaters, I have
already said that he made himself an enemy in
Voldemort's eyes. Karkaroff was forced to fly from the
country the night that the Dark Mark returned, fearing
the Dark Lord's vengeance. Nothing else about this man
is disclosed, save that he has black hair and has a
goatee that he constantly twists around his finger.

Thanks to Mark Gallanger for the Bio!!