Bellatrix Lestrange is exclusive to the upcoming LEGO Harry Potter set 4840, The Burrow. This will be the first time Bellatrix will have appeared in the LEGO universe.
Minifigure Parts and Accessories
Bellatrix is composed of four parts: a hairpiece, head, torso, and slope brick with dress print.
The hair piece is truly spectacular. Almost a sculpture in and of itself, this is a brand new hair style not previously available. This wild style is fairly faithful to the film version of Ms. Lestrange.
The head also features a new print and is also fairly faithful to the celluloid character. The head is dual sided, based on images seen in the Fall 2010 LEGO catalog. The expression seems to be of worry or shock.
The torso and slope brick combine to comprise Bellatrix's dress. The overall design is fantastic and really has a gothic feel. It's very believable that this frock would hang in a female Death Eater's wardrobe. The slope is a variant of part 3678, commonly used for similar dress designs.
As with many of the minifigures wizards in this line, the accessory is a black 4L bar (part 30374) standing in for her wand.
Army Building Potential
I don't see the entire figure herself being sought after, but individual parts could be. The hair, head, and dress have a high degree of custom possibilities. Net/net, I would mark this figure with an ABP of moderate; you won't have armies of Bella amassing, but the individual bits might be sought in multiples.
The main figure parts are the draw here: hair, head, and torso/dress combo. I have started to notice that any female oriented minifigure parts are often sought after, likely due to the fact that male figs are often the norm.
(Pictures from Eurobricks and Harry Potter Wiki)