The Book of Mazarbul was book found by the companions of the Fellowship of the Ring in the Chamber of Records in Moria. It was a sort of diary, in which entries were made by various dwarves, recording the fate of the party which had attempted to recolonise Moria in TA 2989. The last entry ended in the ominous words;
"We cannot get out. The end comes,...drums, drums in the deep...they are coming"References: Lord of the Rings