I assumed that the creature in darkshore had been excavated and moved.

The ancient of lore in Darkshore that sends you there says in the quest text that it was a servant of the old gods.

Actually no, it doesn't. It mentions the Twilight Cult serves. the "old lords" and that the masters Glaive is a place where an old lord fell. www.wowwiki.com/Quest:The_Master's_Glaive

The thing in Master's Glaive can't possibly be an old god. There's an identical creature with a glaive in its head in the Twilight Highlands, and it too is being excavated. Also, they appear to be in different stages of excavation. Unless they excavated the thing at Master's Glaive, moved it across the planet to the Twilight Highlands, then buried it again so that they could dig it out, they are two seperate entities. This supports that these are servants of the old gods, not the old gods themselves. Whether this "Slitherer" is a new old god that we've yet to see or the name of a powerful old god servant remains to be seen, but I think it's safe to say at this point that the Master's Glaive is not an old god.

You're right, actually; the creature at Master's Glaive, Soggoth the Slitherer, as a servant of the Old Gods. The Titans sent 20 stone giants after him; all but one died, and the last one got lucky when he jammed his sword into his head.

I'll find the quest that tells this on cata.WoWHead later

Its definitely Soggoth the Slitherer in Darkshore.

I played through that questline on the beta yesterday (and its a truly awesome questline).

(DO NOT READ ANY FURTHER if you don't want to be spoiled)

Its an epic questline that has you fight the "Battle for Darkshore" against the Twilight's Hammer, and its a phased quest. You first spy on the Twilight's Hammer and learn that they are indeed, trying to summon Soggoth. The whole area has changed now, and theres a lot of 'faceless ones' wandering about with the twilight's hammer.

The druids at the Grove of the Ancients send you to Remtravel to learn what you can about Soggoth; apparently he was originally taken out by 20 Stone giants sent by the titans. All giants but 1 were lost, and the last giant allowed Soggoth to wrap him in his tentacles and lift him up, so that he could sink that giant sword into Soggoths skull.

The druids are a bit nervous with this information, but decide that instead of stone giants, the Ancients could help (There's a quirky description of them as 'lumber giants'). To rouse the Ancients from their slumber, you need to get an artifact, but the keeper of the grove has been murdered... by the Naga! (yes, its really strongly implied that the Naga are involved with the Twilights Hammer somehow)

So, off you go offshore to confront the Naga in a cave, only to find out that their leader has sent his summoners up top to present the item to Azshara herself.
As you make your way up to try to stop her, Malfurion appears to help you out leading to an angry exchange between himself and Azshara. I can't remember much of what was said, i was too busy picking my jaw up off the floor at this point. :D

In the meantime, the Twilights hammer have sped up their plans, but because they've sped it up, they've summoned Soggoth only as an Avatar of Soggoth. You then call on the Ancients, and they appear to help you take down the Avatar of Soggoth

It was an unreal questline. I'm not entirely certain on whether it begins in the Grove, with Malfurion himself a little bit earlier, at the Eye of the Storm (thats another really epic one) Everyone should definitely try them both!!! :D