Nathros' axe is sufficient to end the crawler, thought that's not immediately clear given its thrashing and tentacle-spasms. It's clear in moments though, as the additional blows rain down on the beast. By the time Alastor raises his lamp, the thing is a lifeless worm. It only mostly blocks the tunnel, and one could squeeze by along the right-hand wall.

Mycah and Artanis perch on the edge of the ledge, the party below them and the tunnel ahead.