While they may not have been the most powerful or terrifying followers of the Dark Lord Sauron, the Orcs made up the largest faction of his army. According to The Silmarillion, Morgoth (the first Dark Lord) had corrupted some of the Elves in mockery before torturing and deforming them at his fortress of Utumno until they became the first Orcs. Described as Morgoth’s “vilest deed”, they were bred in thousands to fight his battles and support his invasion of Middle-earth. The Orcs continued to do the same for Sauron from the Second Age onwards, forming the backbone of his army. Even stronger breeds emerged over time, including the Uruk-hai and Black Uruks.
At the end of the Third Age, Sauron’s defeat in the War of the Ring raises the question: what happened to the vast army of the cruelest and most sadistic creatures to exist in The Lord of the Rings?
How Was Sauron Defeated?
Like Gandalf the Grey, Sauron was a Maia — hence, he could not completely perish like the mortal beings of Middle-earth. However, unlike Gandalf (who was reborn as Gandalf the White), he did not return to Middle-earth after his final defeat. This is because his fate was tied with that of the One Ring; as long as it remained intact, he would continue to exist in some form or the other.
This can be seen on multiple occasions during the Second Age. Intent on taking over Middle-earth, Sauron deceived the Elves in creating the Rings of Power while secretly forging the One Ring in Mount Doom to rule the other rings. When the Elves learned of his deception, they removed their rings and hid them. This eventually led to the War of the Elves and Sauron. Even though he managed to conquer most of Middle-earth during this time period, Sauron ultimately faced defeat in the Battle of the Gwathló. He spent the next few centuries rebuilding his strength until he could gather a large and powerful army for his purpose once again. After the downfall of Númenor, the Elves and Men united against Sauron in the War of the Last Alliance. It was during the siege of Barad-dûr that Isildur, the son of Elendil (the High King of the Dúnedain), was able to cut the Ring from Sauron’s finger with his father’s sword — reducing Sauron to a non-corporeal state. Even though his power had been greatly weakened, he still existed as Isildur chose to keep the Ring instead of throwing it into Mount Doom. It is the Orcs who eventually killed Isildur, and the Ring was lost somewhere in the Gladden Fields for around 2000 years.
Sauron spent the next 1000 years in hiding, trying to regain his physical body. He openly began to prepare for his final war in 2951, and later learned from Gollum that the One Ring is in the possession of someone called Baggins. The War of the Ring took place just 2 years later.
Being in possession of the Ring at that time, the Hobbit Frodo Baggins planned to travel to Mount Doom to destroy it, but did not know how to reach there unnoticed by Sauron. As a result, Gandalf suggested that the Army of the West should march towards the Black Gate of Mordor to distract Sauron from the Ring-bearer’s movements. His plan worked since Sauron stationed most of his forces at the Black Gate, allowing Frodo and his friend Samwise Gamgee to reach the Cracks of Doom. Sauron realized this far too late — by the time he ordered the Nazgûl to go and get the Ring back, Gollum had snatched it from Frodo and accidentally fallen to his death into the fires of Mount Doom. With the Ring finally destroyed, Sauron lost his physical form permanently this time, and his spirit lingered above Mordor as a “huge shape of shadow… terrible but impotent” before being taken into the Void.
What Happened to the Orcs After the War of the One Ring?
Without the Ring’s power sustaining it anymore, Barad-dûr completely collapsed and Sauron's army fled. The final installment of the film trilogy shows the Orcs either escaping or being swallowed up by the ground beneath them after the war ends. Since their will was not bound to Sauron now, it is more likely that they ran away in terror and scattered across Middle-earth. After Sauron’s defeat, Aragorn and his army killed the remaining Orcs in Mordor. It can be assumed that any Orcs in the Lonely Mountain would have also escaped once they got to know about their master’s downfall.
Although the author J. R. R. Tolkien does not mention them in the Fourth Age, it is likely that any surviving Orcs had also been hunted down by other species (most probably Men) by then. They rarely lived beyond 100 years of age anyway plus, as female Orcs seemed to be missing throughout the series, there is uncertainty about whether they were able to procreate instead of becoming extinct.