A Treant with Bewbs....Not So Much (II)

Apropos of my post yesterday, a short Monster Idea:

Knocker

The Knocker is a subterranean spirit which inhabits the deepest parts of mine shafts and dungeons. In appearance it resembles a very small, beardless dwarf, but it is never seen - only heard. The only contact an adventurer is likely to have with a Knocker is with its scampering footsteps and the repetitive tapping in the darkness which gives it its name.

Knockers' behaviour is impossible to predict. Half of the time they are a mysterious but benign force, alerting dungeon delvers and miners to the danger of a tunnel collapse with their tapping. But half of the time they are a malevolent haunting who deliberately cause tunnel collapses to crush those they take against. The DM should roll a percentage die whenever Knockers are encountered to determine whether they are in a helpful or malicious frame of mind. If the former, they will appear and begin tapping to warn the PCs of the presence of a randomly determined trap. If the latter, they will cause a tunnel collapse.

Knocker
Alignment: Chaotic Neutral
Intelligence: Very (11-12)
No. Appearing: 1d12+1
Hit Dice: 2
THAC0: 18
No. of Attacks: 2
Damage/Attack: 2d2
Special Attacks: See below
Special Defenses: See below

Knockers never engage in physical combat unless they are somehow cornered - which is nigh-on impossible as the creatures are incorporeal and can pass through (and remain inside) walls at will. They are never surprised, and can be hit only by magical or cold iron weapons.

A group of one or more Knockers can collectively use a special ability, Cause Tunnel Collapse, once per day. They can knock for as many rounds as they wish, during which time the potential collapse will gradually grow in size; as soon as they have stopped knocking the collapse occurs, causing 8d6 hit points of damage over an area 15' in size per round knocked, although a successful save vs. breath weapon will half the damage caused. Those caught in the collapse must also save vs. paralyzation - if they fail they become trapped under rubble and suffer 1d6 per round in crushing damage until they are freed (takes 1d6 rounds). A tunnel collapse generally destroys a section of the tunnel equivalent in size to the 'knocked' area and cannot generally be cleared without magic such as pass wall or stoneshape.