Siege of Barad-dûr (3434 S.A.) - The final battle that brought about an end to the Second Age.  It featured the last alliance of Elves and Men against the forces of Mordor, and would last seven years.

 

Sauron survived the Fall of Númenor because he possessed the Ring of Power, but the catastrophe did not leave him unscathed.

 

"...his seat and his temple fell into the abyss.  But Sauron was not of mortal flesh, and though he was robbed now of his shape in which he had wrought so great an evil, so that he could never again appear fair in the eyes of Men; yet his spirit arose out of the deep and passed as a shadow and a black wind over the sea and came back to Middle-earth and to Mordor that was his home.  There he took up again his great Ring in Barad-dûr, and dwelt there, dark and silent, until he wrought himself a new guise, an image of malice and hatred made visible..." ~The Silmarillion (Akallabêth)

 

The free peoples' of Middle Earth saw an opportunity to defeat their Enemy once and for all.  And they did defeat Sauron (temporarily), but it did not come without a cost.

 

"'It was Gil-gald, Elven-king and Elendil of Westernese who overthrew Sauron, though they themselves perished in the deed; and Isildur Elendil's son cut the Ring from Sauron's hand and took it for his own.  Then Sauron was vanquished and his spirit fled and was hidden for long years...'" ~The Fellowship of the Ring, Book I, chap. 2 (The Shadow of the Past)