The name 'Haunting Custodian' comes from AD&D but I always found the creature (aka the 'Drelb') somewhat dull. Here's my take on a creature of this name:
No. Enc: 1
Damage: 1d4 + root
Haunting Custodians, also known as Drelb, are a type of earth elemental summoned and bound to a particular building or dungeon to maintain it. They 'put things to right', resetting traps, closing doors, repairing damage and generally trying to keep things as they are.
These creatures appear as 4' tall stone humanoids with brass beards. Though quite strong they do not use weapons, and can only strike with a fist in combat. However anyone standing on stone who is struck by one must save vs. magic or have the stone flow up over their lower legs, rooting them in place until the stone can be broken away (8 points damage from blunt weapons or picks, any excess damage being taken by the trapped creature). They generally do not fight in any case, as they can move through stone as easily as air and tend to avoid confrontations.
Drelb can cause a variety of sounds to emanate from stone that they are within. Knocking, tapping, even their rasping voice. They will use this ability to annoy or disturb interlopers, or to draw dungeon dwellers to various locations, usually with a view toward making trouble for destructive invaders.
As far as custodial functions, consider a drelb to be able to accomplish the work of five skilled and equipped miners or stonemasons.
Binding a drelb to service requires a fourth level MU spell. The spell requires that the Custodian's 'pay'--one 100 gpv gem per year of service--be placed in an elaborately cast and carved stone and metal vessel that is placed inside a magic circle in a room in the place to be guarded. The magic allows the drelb to remove a single gem per year from this vessel. (More can be added to extend the term of the binding.) If the magic circle is broken so is the Custodian's binding. If freed it will often try to seize whatever of it's 'pay' remains before departing forever.