From the SRD:
At 3rd level, a shadowdancer can summon a shadow, an undead shade. Unlike a normal shadow, this shadow’s alignment matches that of the shadowdancer, and the creature cannot create spawn. The summoned shadow cannot be turned, rebuked, or commanded by any third party. This shadow serves as a companion to the shadowdancer and can communicate intelligibly with the shadowdancer.
I think this is capital-S "Shadow", undead. I'd use the Shadow entry from the MM, and update alignment, special abilities etc. as per the description above.
file:///Y:/Web_Sites/soveillor_srd/geocities.com/sovelior/srd/monstersS.html#shadow
