This character is simply, a raised death knight, dressed up as a witch for the victory against the Scourge. Strangely enough, the history goes back to level 55 when death knights help the Scourge to eliminate everyone in Lich King's way, to the present, where we serve as the Alliance or the Horde and as most guilds, stand
against the Lich King. This character redeemed consciousness when the Lich King's control over his champions was broken, and avenged its death the same way. Most of the players don't notice this when they play death knights, story that might seem depressing in a way.
"A thousand atrocities are etched in the eyes of every death knight, and foes who gaze into them too long will feel the warmth pulled from their bodies, replaced with cold steel."
I'm a big scary witch that likes to take loot
when the raid doesn't have an enchanter
saving Azeroth from Headless Horseman,
or that "pug" that wiped us last night
I'm a big scary witch
that used to be <the Scourge>* itself
serving Lich King in the Acherus
twenty-five bloody damned levels ago
*guild <The Scourge>
I'm a little scary witch