Lay of Leithian: Canto Nine

We view the great rescue of Beren by Lúthien and Huan in canto nine. First Felagund sacrifices his life (only he and Beren remain of the 12 who left Nargothrond) to save Beren from the jaws of a wolf. As Beren chants fighting songs over his dead body Lúthien and Haun arrive and she begins to sing back. This causes Thú to dispatch his wolves, Draugluin included, included. Each was slain by Huan. Lastly Thû took wolf shape thinking in his pride he would kill Huan but the hound of Valinor was not to meet his end this day. Capturing Thû in a death vice Lúthien demanded the key to his fortress:

"'O demon dark, O phantom vile

of foulness wrought, of lies and guile, 

here shalt thou die, thy spirit roam

quaking back to thy master's home

his scorn and fury to endure;

thee he will in the bowels immure

of groaning earth, and in a hole 

everlastingly thy naked soul

shall wail and gibber--this shall be,

unless the keys thou render me

of the black fortress, and the spell

that bindeth stone to stone thou tell,

and speak the words of opening.'"

(p. 253-254, The Lays of Beleriand)

Thû gave up the spells and Lúthien cast down the walls enabling her to reunite with Beren, her love, when all hope seemed to be gone. 

As an aside, in the midst of the walls coming down it is told that Thû took bat form and flew to Taur-na-Fuin, the forest of Deadly Nightshade, to build a stronghold there. This is a connection to The Hobbit which Professor Tolkien would have been writing in approximately the same time frame as The Lay of Leithian. How deliberate the connection cannot be said as Tolkien often borrowed from himself when writing his stories. But Taur-na-Fuin bears a striking resemblance to Mirkwood as does the evil wizard Thû to the Necromancer (later Sauron). It can be seen that Mr. Baggins of Bag End may have walked to the brink of the Eldar Days along his way to the edge of the Wild.