Caveman wrote:

[Yeah saw a fossil on TV which was curled like that,  and just over a foot diameter (in RL), and thought it looked like a worm. . If ya stretch that curl out, you get about 15 ft. Good of it to squeeze through a 2x2 feet hole. ]

Narrhil waits, he would not risk another venture till he knew the way was safe. If the Hounds were as patient as him, they would wait for days, weeks but he had plenty of food to sustain himself, plus the great worm to protect him when he sleeps.

Nothing emerges from the hole. Silence presses Narrhil's ears.
The worm returns to peaceful sleep. The seconds become minutes.
Narrhil's wand arm tires. He fears Tillak is dead. 

More minutes pass. The worm remains unconcerned.

Narrhil is singed. His beard mere tuft.
His skin enflamed to the touch but not seared.
An ointment would go a long way....

Nothing is moving at the hole opening.
Tillak remains motionless and mute as well.
The corpse has deteriorated quite a bit since the ring was removed.
The worm slumbers.
The stone near the opening is affixed to metal that can be aligned into a track that from this angle appears to you allows to to slide in and out of the crawlspace.
The low surface of the dome restricts full erect standing. 
And you see what looks like a small pin at the apex of the dome.

 

He would have to cross the opening and figure out how to put the stone back into track, the stone is slightly smaller than the opening. He cannot tell how it works.
The pin is metal. One inch.
More minutes pass. The hound does not emerge.