Within The Lord of the Rings and the other stories set in Middle-earth, the corrupting power of Sauron is a legendary bane for the world. Various beings have been broken and twisted by his influence, making up some of the story's most dangerous figures. However, it may have extended even further than anyone realized.
A Lord of the Rings fan theory surrounding the Ents might explain whatever happened to the female members of their species, known as Entwives, and how they became corrupted into some of Sauron's most deadly warriors.
The backstory devised by J.R.R. Tolkien for the Ents suggested the Entwives had come into being alongside their male counterparts. However, sometime during the Second Age (thousands of years before the events of The Lord of the Rings trilogy), the Entwives departed from the rest of their kin. They ultimately relocated to the Brown Lands, leaving them open to an assault from Sauron when he began to make his move to take control of Middle-earth.
The Entwives were lost to mystery in this period, with even Tolkien himself noting in a letter (later collected in The Letters of J. R. R. Tolkien) that he intended for there to always be a question about their fate. In his estimation, the Entwives were likely destroyed along with their gardens in the lead-up to the Last Alliance's grand battle with Sauron, meaning it may be one of the events covered in Prime Video's upcoming The Lord of the Rings: The Rings of Power series. But one fan theory from Reddit user Harboringonalament hints at a more tragic fate for the Entwives that ensures they lived on in a new, twisted form.
As Harboringonalament explains, the Shadow Lands were among the locations overtaken by Sauron's forces and reduced to ash and ruin in the Second Age. But if the Entwives had been merely killed, there might have been something that remained of them for someone like Treebeard to discover. However, their complete absence from the world suggests something else. Given Sauron's habit of corrupting species -- such as creating orcs from captured Elves or turning human kings into his Ringwraiths -- it wouldn't be shocking to discover he'd done so to the Entwives as well. In fact, this could secretly be the origins of the trolls.
Although many trolls are said to have been created by Melkor (the first Dark Lord and a precursor to Sauron), his lack of the Secret Fire meant that he could only corrupt other beings, suggesting the trolls were once a different species. Modern trolls could be the Entwives, corrupted by Sauron. Treebeard specifically notes in the original book version of The Two Towers that "trolls are only counterfeits, made by the Enemy in the Great Darkness, in mockery of Ents, as Orcs were of Elves." The orcs are corrupted versions of Elves, hinting such a connection might exist between the Ents and the trolls as well.
The theory suggests that during the Second Age, Sauron captured the Entwives and corrupted some -- or even all of them -- into trolls as he spread his influence through the Brown Lands. Any that weren't transformed were destroyed. This means the beings that were once the Entwives might have quietly been involved in the series all along, threatening the heroes in various battles across Middle-earth and even coming close to killing Frodo and Aragorn at different points in the film trilogy.
If true, this would be one of the cruelest acts of Sauron in a life full of such atrocities. By fully corrupting the Entwives and turning them into largely mindless killers for his armies, he truly contrasted them against the remaining Ents, who remained largely peaceful and unconnected to many of the battles that came to define the Second and Third Ages.
Sauron's evils are often shown reducing a vibrant and natural land into a grim and grey one, and by corrupting the Entwives and stopping the spread of the species, he may have done so while adding powerhouse combatants to his armies.
While the answer will likely never be truly known -- with Tolkien himself wanting that question to never be formally answered -- it's an interesting concept to consider, especially in light of the Ents' decision to take part in the War of the Ring and ultimately defeat Sauron's renewed plans numerous centuries after the loss of the Entwives. Without realizing it, they may have gotten revenge for the decimation of their own people at his hand.