Devised in the second but died out there as well
revived in the third the history does tell
Used by those who stood in highest stead
Their words alone brought the direst dread
(Jeremy Richman) Thanks to Scott Baltes for this. The correct answer is "Black speech" -- this was invented (devised) in the Second Age by Sauron, and I believe used by the Nazgul in the LOTR.
Oathbreakers?
(Jordi Chalmeta) Oathbreakers did not work.
The Oathbreakers, also known as the Dead Men of Dunharrow, were the shades of Men of the White Mountains.
Tolkien Gateway: Oathbreakers