This has always been one of my favorite images in D&D:
It's completely awesome. Eyes peering out madly, strange beings sallying forth but still connected to the main being (including a member of the cone-shaped race inhabited for a time by the Great Race of Yith, there in the back up the stairs) . . .
At the same time, I'm not even sure how I'd run that. I could describe it, but it would be hard to keep having it spew out new beings constantly. You'd probably need to use a random generator to spit out stats for the "spawn" that keep coming out to fight and die on the blades of those trying to banish some bit of Shub-Niggurath back to where it came from.
It could be a great set-piece for a high-level AD&D game - you've got stats right there in the old edition of Deities & Demigods - or as a DF ultimate boss monster. It's hard not to think of ways to try and use this, thanks to the great feel of Erol Otus's art.