Challenging undead for first levels?

[CKDad]

To set the stage: my players (all 11-12 year old boys) are about to pursue a band of orcs down into the crypts beneath a castle, which the orcs had tunneled into so as to take the castle from the inside. The PCs are in the process of re-securing the castle, and the orcs will retreat the way they came, down into the undercroft and then to the crypts enroute to series of caves and tunnels.

My plan (should it survive contact with reality!) is that at some point in the running battle in the crypts, one of the orcs will accidentally trip the mechanism for a secret passage - and some undead, lurking within, will join the fray. Three-way furball battle time!

Skeletons will form the main group of undead, but I think the situation calls for a climactic encounter - a mini boss-fight, if you will. My puzzle is what to use as the mini-"Big Bad" undead. At the moment, I'm considering a down-powered ghast or wight. A wight would fit better thematically, and down-powered a bit (say, two or three HD instead of four, either lose the energy drain or reduce it's potency) it should present a significant challenge.

Am I overthinking things? Is that perhaps too much to throw at a bunch of relatively new, young players? Some other undead beastie I've overlooked?

I do have MM1 and the Fiend Folio available for possible conversion fodder, but I'm open to suggestions from elsewhere.

Thanks in advance!

[serleran]

I would use a neo-wraith, which resembles a shadow but has all the properties of a wraith... at one-quarter strength.
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