I think it would depend on the cosmology you want to use. With the default D&D cosmology, I think shadar-kai elves are the closest match.
In Mystara, there isn't really a Shadowfell, or even a "plane of shadow" so to speak. The closest you would get is probably Limbo (the "Realm of the Dead", probably best described in
Dragon Magazine #180) or the Land of Mists (from adventure module
X2: Castle Amber). If your game uses this cosmology, you might consider using tieflings, dhampir, hexbloods, and The Reborn in addition to shadar-kai.
Which cosmology is your game using,
@Guang ?