Towards the Hoard of a Hundred Horrors: Wightsnakes

There's snakes in these here parts, horrible snakes, yellowed-off white like old bones - they don't swallow their prey live like other snakes do, they like them ripely putrid, holding the strangled corpses tenderly in their coils until they start to reek, and leak.

There's barrows too, old and haunted (of course) - and they hold the most delectable corpses of all (to the snakes). But to end up in a snake's belly is the foulest defilement of them all, so the barrows' stones and walking bones endeavour to keep the reptiles out and far away.

But sometimes a snake has its day, and seizes its well-aged barrow-corpse delicacy. Then it is reborn - the merger of ancient hero (in the classical sense) and serpentine voracity.

They are

Wightsnakes

HD: 6 AC: As armour in human form, 15 in snake form ATK: As weapon in human form, Constrict and 1d4 bite plus Haunting Venom in snake form SAV:12 MOV: As human in human form, as big snake in snake form INT: As classically-educated person ML: 8
No. Appearing: 1d3

Shapeshift: Once per round, as a free action, a wightsnake can shift between their human and snake form.

Constrict: A target hit by constrict is automatically grappled by the wightsnake. A wightsnake can constrict an already-grappled target to automatically deal 1d8 damage to them. It takes a combined strength of 30 to pry a wightsnake off a grappled target. Attacks aimed at a constricting wightsnake have a 2-in-6 chance of hitting their constricted target. A wightsnake can only constrict one target at a time.

Heroic Panolpy: If you roll up a wightsnake with more than 21 HP then they've got a magic weapon. If you roll up a wightsnake with more than 26 HP they've got a magic weapon and a piece of magic armour. 

Haunting Venom: A wightsnake's venom is full of ghosts. If bit, save or roll on the table below (1d6):
1. Necrosis: Take an immediate extra 1d6 damage.
2. Hungry Ghost: Ordinary food won't sustain you - only offerings left for the dead will. Save again every day thereafter or until the poison is purged.
3. Possession: Randomly change your character to another class and personality. Save again every day thereafter or until the poison is purged.
4. Marked by the Grave: You are affected by magic as if you were a corpse rather than a living thing. Save again every day thereafter or until the poison is purged.
5. Poltergeist: A small item in the vicinity is flung at a random member of your party. Save again every hour or it happens again.
6. Fearsome Moaning: You will not be able to sleep restfully that night. Save every night thereafter until you succeed or the poison is purged.