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The Abyssal Maw is a scrapped instance from World of Warcraft: Cataclysm, originally planned for patch 4.1 alongside the Firelands raid.

It was initially pushed back to patch 4.2 along with the Firelands, then scrapped entirely to focus patch 4.2 entirely on the Firelands, reasoning that for the time being Throne of the Tides is a good ending to the Vashj'ir storyline. So Abyssal Maw was supposed to be released with patch 4.1 before it was cancelled and replaced with Zul'Gurub and Zul'Aman.

However, fans have noted that it ends on a vague (and hard to see) cliffhanger where Ozumat kidnaps Neptulon. In regards to this at the BlizzCon 2011 World of Warcraft Lore and Story Q&A panel, Metzen proceeded to dodge the question with jokes about "Neptulon's tryst with Ozumat" and said that it was a hard question and that a "damn mess is what it was.

The fights would have taken place inside a third shelled demigod that had been corrupted by the naga. Ghostcrawler says that "It was three bosses inside Nespirah, with no unique art," though earlier previews described the dungeon as multiple raised underwater plateaus above a chasm.

The Abyssal maw dungeon was scrapped because the storyline and setting was not convincing and fitting.

While it was originally announced as a 5-man, later statements suggested it may have been changed to a raid before getting scrapped.
Ghostcrawler about the instance

Ghostcrawler talked about the Abyssal Maw during Blizzard's Cataclysm post-mortem blog series.

I feel like I should mention Abyssal Maw again. As with many cancelled features, it somehow took on a life of its own in the minds of players. Believe me, though -- you just don't cancel things that you think are going to be awesome. It was three bosses inside Nespirah, with no unique art. The reason it was originally appealing to us was because we had so many Vashj'ir assets that we could use. But by the time it was time to do the work, we felt like we (and many players) had Vashj'ir fatigue. Now don't get me wrong -- I loved Vashj'ir. I was an oceanographer, remember? Vashj'ir delivered on the promise of an underwater zone, but we feel like most players were ready to be done with it by the time they had quested through that. (Individuals will feel differently -- it's that diverse player base thing again.) Firelands received a lot of new art, from bosses to environments, and we just didn't feel like Abyssal Maw was going to compete. Who knows, though! We haven't totally given up on the idea of cool underwater experiences, so maybe there's potential we'd visit it again someday. (For my money, the zone I am personally saddest about cancelling is not Abyssal Maw; it was the Azjol-Nerub quest zone in Wrath of the Lich King.)
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