A single stone out of place is all it takes. A tendril of darkness will worm it's way in, and coil itself around the corpse like a lover. Such a tether denies the spirit rest. Beneath the barrows, they forget who they were. The honest mistake of a twisted stone becomes a deliberate betrayal.
Draped in their funeral panoply they emerge under cover of night - seeking those who denied them rest. Sunlight banishes them to their tomb - and opening that to the sky would destroy them utterly.
They are careful to never be seen emerging - and to open all of the barrows is sure to invite more restless dead.
Those they slay are bound to them as thralls, and will rise with them every night - a poor imitation of what they were promised in death.
HD 2* / AC as Unarmoured / Damage as Weapon, applied to Constitution.
*Incorporeal - Cannot be harmed bar by Silver Weapons (Quadruple price, -1 to hit corporeal foes) and Magic.