It!

Here's the original muck-encrusted mockery of a man, from Theodore Sturgeon's 1940 tale, as adapted here by Roy Thomas, Marie Severin & Frank Giacoia. We actually had the original story in a sci-fi / horror compendium in our school library, and I vividly remember reading It! one lunchtime, and being so repulsed by Sturgeon's descriptions of the creature that I couldn't finish my sandwiches...
Even with decades of Man-Thing's, Swamp Thing's, Bog Beast's and Heap's, It! is still powerful stuff, Marie and Frankie are on top form as always, and Roy keeps it all as nasty as the original.
Astoundingly, someone suggested at the time that It should have it's own series, just at the same time that Manny was slithering out of the swamp. Possibly, that's the apocryphal tale of Len Wein & Marv Wolfman trying to convince Stan to do a female version called She-It ( say it quickly ). Anyway, common sense prevailed. This is an absolute one-off.

The Valley Of The Worm

One of Robert E. Howard's greatest one-off stories was actually adapted twice into comic form.
In 1975 John Jakes & Richard Corben opened up and expanded The Valley Of The Worm into the post-apocalyptic graphic novel Bloodstar, but Marvel got there a year earlier in this issue of Supernatural Thrillers.
Both versions are, of course, magnificent, so let's start with the short version, courtesy of Roy Thomas, Gerry Conway, Gil Kane & Ernie Chan.
Roy and Gerry, natch, absolutely catch the flavour of REH's prose and two-fisted poetry, and if I'd prefer, say, Dan Adkins sleeker inks over Gil's pencils, you can't deny Ernie adds a certain grittiness to the art.
Like a lot of REH's work, Valley Of The Worm works with the concept of ancestral memory and past lives, adding an extra edge to the tale, and James Allison / Niord is as doom laden as the best of his heroes. In fact, you can easily see Bran Mak Morn singing his death song before going into battle, as Niord the Aasgardian does here, not to mention sidekick Grom being an ancestor of Brule The Spear Slayer.