1 Lambert: Gelatinous cube.
1 Draak: Hag.
1 Dwalin: Unvusible stalker.
2 Mordekai: Oh, what? J?! Why do I have to get J? What monsters start with J??
2 Lambert: Are you out then?
3 Mordekai: No, wait. Juiblex!
3 Lambert: What did I say about demon lords?!
4 Mordekai: Oh come on. What are the odds that the Faceless Lord of Slime will bother manifesting for us?
4 {a faceless mound of slime manifests behind Mordekai}

The gelatinous cube, unlike the flumph, is one of my all-time favourite D&D monsters. Admittedly it's only slightly less ridiculous than the flumph, but there's just something delightful about a huge cuboid mass of jelly. You can read about them on Wikipedia (academic and boring) and Forgotten Realms Wiki (in-world, and with pictures!).

Hags are, well, hags. In this case the Wikipedia page is more interesting than the Forgotten Realms Wiki page.

Invisible stalkers are... pretty much exactly as described on the tin. Check Wikipedia or FG Wiki.

And finally, Juiblex is, as Lambert and Mordekai convey, the Demon Lord of Slimes and oozes. Again, Wikipedia and FG Wiki.

A reader sent me the ooze miniature many years ago. It's good to get to use it in a comic.