Third Siege of the Black Gate

The Third Siege of the Black Gate was an inevitability; rather, the Mallorn Council decided that one defender was enough so as to not waste the resources of the rest of the Council. Legolas quickly volunteered, and the battle was won by Sauron with ease.

Prelude
The Second Siege of the Black Gate had severely limited the Mallorn Council's ability to defend the Morannon, and thus Legolas had volunteered to defend the Black Gate alone. The plan was simply to do as much damage to Sauron's forces before retreating, then attacking at a later date.

Battle
The battle went completely in Sauron's favor, as a single Olog-hai swung his hammer and knocked every last elf clean of the Black Gate. Many died from their fall, while others were quickly picked off by the Orcs below. Legolas managed to flee with a few of his loyal elves, and Sauron retook the Black Gate and greatly bolstered its garrison.

Aftermath
Though the battle went as expected, the Mallorn Council had hoped the Elves could damage Sauron's numbers more. Sauron's first move after reclaiming the Black Gate was to bolster its garrisons, making the fortification nearly impregnable. This proved to be a thorn in the Mallorn Council's invasion plans, leading them to be repelled several times in the future.