One of the PCs in my DF Felltower game - Gerry* - has the Skull Spirit spell, and a fair number of skulls he's collected to use it.
Here is what we use for stats for the created ones:
ST 0 HP 20 Speed 6.00
DX 14 Per 10 Move 12 (Air)
IQ 10 Will 10
HT 10 FP N/A SM -5
Dodge 9 Parry N/A DR 0
Ghostly Touch (14): 2 points toxic. This attack ignores all DR! Cannot be parried or blocked. Reach C.
Traits: Aerial; Dark Vision; Doesn't Breathe; Doesn't Eat or Drink; Doesn't Sleep; Flight; High Pain Threshold; Injury Tolerance (Diffuse); Immunity to Disease; Immunity to Mind Control; Immunity to Poison; Indomitable; Intolerance (the Living); Mute; No Manipulators; Slave Mentality; Unfazeable; Unnatural (dies at -1 x HP).
Notes: Glows whitish-green equivalent to torchlight. Will obey its creator only, and will not take commands from others. Usually tasked with a specific goal - kill a specific target. Lasts 24 hours before it expires. Truly evil.
* Of Gerry & the Skeletons fame.