These guys aren't making much use of their spell. It never webs anything. The shadow web vanishes shortly after they cast. Not that scary if you ask me!

One of the emaciated umbramagus's heads snaps out a guttural phrase while the other chatters a rhythmic pattern on its teeth!
A spiraling pattern of deep ebony jets outward from the umbramagus's spindly arms!
Dark cracks interlace themselves across the ground, snaking in rapid succession until they coalesce into a writhing web of living shadows!
>look
[Subterranean Depths, Limestone Strata]
Deepening shadows and echoing footfalls flow eerily behind each step around the curvature of the narrowing cavern walls. A distant roar rises upon the still air filling the tunnel then lessens steadily into silence before crashing once again far below. With each thunderous rush, dense mist fills the air momentarily, causing the smooth rock underfoot to become even more hazardous than before. You also see a writhing web of shadows, a shadefield eviscerator and an emaciated umbramagus that is in a globe of blue fire.
Obvious exits: east, southwest.

The web of shadows twitches one last time and then goes inert.