Thangorodrim - "Mountains of Tyranny".

"And he (Morgoth) being freed gathered again all his servants that he could find, and came to the ruins of Angband.  There he delved anew his vast vaults and dungeons, and above their gates he reared the threefold peaks of Thangorodrim, and a great reek of dark smoke was ever wreathed about them.  There countless became the hosts of his beasts and his demons, and the race of the Orcs, bred long before, grew and multiplied in the bowels of the earth." ~The Silmarillion, chap. 9 (Of the Flight of the Noldor)