In The Fellowship of the Ring:
While Gandalf is figuring out how to open the Hollin Gate into Moria, Merry and Pippin toss stones into the lake nearby. The lake had only in the past few years risen so close to the entrance to Moria. Aragorn advises them to stop, but it is too late. Something has begun to stir in the lake. Meanwhile, Gandalf has opened the door and The Fellowship enters Moria. Once inside, they discover dead Dwarves and orcish weapons. They flee back outside.
Something slithers out of the lake and grabs Frodo. The Fellowship manage to rescue Frodo and flee back into Moria as the creature slams the doors shut and then piles boulders outside blocking the entrance behind them. The creature's origins are vague and like the Balrog is not a servant of Sauron. However, it is interesting that in its malice, it targeted Frodo, who is the Ringbearer.
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