Hungry Coffins

Up ahead, you see a quartet of tombstones, right in the middle of the road.  For about a foot around each tombstone, the dirt looks fresh.  The headstones have a skull at the top, and a decorative pair of small stone wings.

The tombstones begin to rise, and dirt rains off three stone coffins, now hovering in the air.  The tombstones crown the coffins, which were buried vertically (with any corpse inside in a standing position).

Then the flying coffins pop open, each disgorging a zombie.  Their interiors are lined with spikes, like an iron maiden.  The zombie attack, as zombies, do.  At their crown, the tombstone goes blank.

Then the flying coffins attack as well.  One of them flies above a PC, then drops, trying to pulp them into the ground like a giant's hammer.  The other two begin snapping their lids, trying to snatch up a PC into their interior.

One of them catches a PC, who is impaled inside the closing coffin.  Then the coffin plunges into the earth, which parts to accommodate the hungry coffin.  After the PC inside is dead, writing appears on the headstone, accurately reflecting the PC's name, birth and death dates, and their last words (in quotes).

Despite their appearance, hungry coffins are actually gargoyles suffering from a specific type of curse.  If remove curse is cast on them, they turn into normal gargoyles (HD 4) and are stunned for 1 round.

from Super Ghouls and Ghosts
the coffins actually do fly, briefly,
before dropping a zombie and vanishing

Hungry Coffin

HD 6 AC 14 Slam 2d6 or 2d8 Swallow 1d6 + swallow
Fly 9 Int 5 Morale 12
* Immune to slashing, piercing, fire, and lightning.
* Buried coffins always contain a zombie (HD 2) which they can disgorge as a standard action.
* Slams deal 2d6 damage if the coffin is empty, or 2d8 damage if they contain a person (or zombie).
* Swallowed creatures are trapped inside the coffin.  They take 1d6 damage whenever they end a turn inside a coffin as the spikes twist in their flesh.  Swallowed creatures can break free by rolling under their (Strength - 10) or if someone on the outside pries it open (with the same Str check).  Each additional creature or prying instrument gives +4 to this check.
* Coffins that have swallowed a person will try to burrow into the ground as a standard action.  Buried coffins can be excavated after 1d12+1 standard actions have been taken to unearth it.  Shovels allow for excavation at twice the speed.  When buried, a coffin cannot be forced open (as dirt holds the coffin closed).  If you unearth a buried coffin, it is stunned for 1 round before taking to the air again.

If you want to know what a hungry coffin will do in a given round, flip a d2:

1. Slam attack.
2. Disgorge zombie / swallow attack / bury yourself.

Discussion

Think carefully before you put multiple hungry coffins in a single encounter.  The fact that they can swallow you and then bury themselves is really nasty, and the possibility that two coffins might bury two PCs simultaneously means that even high level parties will have a hard time taking the requisite number of standard actions to dig up their friends.

If you want to spice things up, consider putting different types of party favors inside this pinata:

1. ghoul
2. swarm of bats
3. ochre jelly
4. 1d4 halfling zombies
5. poison gas (3d6 damage, Con check for half)
6. miniature hungry coffin (halfling sized) with HD 3 (reduce all damage by 1 die size)