If there are two things that fans of the Lord of the Rings know about Grima Wormtongue, it's that he is willing to betray anyone and everyone to get what he wants, and that he gets very vengeful when he is backed into a corner. When the audience first meets this slimy character, he has all the airs and graces of a noble man of Rohan, portraying himself to those around him as a trusted and loyal advisor to the king: “I only wish to serve you my lord.”
But when the 4 members of the fellowship arrive to find out why Theoden king hasn’t made a stand against the growing number of orcs raiding and burning through Rohan's villages, they soon discover that Grima is not all he seems. Gandalf is able to spot a worm a mile away, and very quickly realizes that Wormtongue isn’t the only cunning and twisted mastermind between Theoden's debilitation, but that the white wizard Saruman is also working behind the scenes. Gandalf is thus able to banish the wizard from Theoden’s thoughts and expose Wormtongue as the traitorous wretch he is. But what would have happened if Gimli, Legolas, Gandalf, and Aragorn had never arrived in Rohan?
For a start, it is highly likely that the War of the Ring would have turned out very differently. Without Theoden leading at the Battle of Helm’s Deep, they never would have been able to wipe out a good portion of the orcs from Saruman’s army. Without Theoden charging to Gondor’s aid, the city of Minas Tirith would have been over-run and countless lives lost. And without Theoden in the battle of Pelennor fields, Eowyn never would have been there to rescue her uncle, and wouldn’t have killed the Fell Beast and the Witch-King of Angmar, who is Sauron’s right-hand man.
All of these things would have had disastrous consequences for the enemies of the evil lord, and would have swayed the war in his favor to the point where Sam and Frodo may never have been able to get the ring to Mount Doom to destroy it, and it likely would have ended up back on Sauron’s re-established physical finger. This would have changed the face of Middle Earth, and the lives of everyone involved, forever.
But there is one character in particular who might have been bound to a life of slavery and torment (and far worse) if the fellowship hadn’t arrived to rescue Theoden: his niece Eowyn. One of the biggest plot points for Grima Wormtongue is his greed and his desire. He paired up with Saruman because he was allowed to name a price, a reward so to speak, for betraying his kingdom and his people. The price he named was Eowyn.
In many exchanges between these two characters, it becomes clear that he lusts after her, and that he is removing all of the allies she has from beneath her feet, like her uncle Theoden, her cousin Theodred, and even her brother Eomer, who Grima has banished from the kingdom. Eowyn is essentially left with no defenses but her own. And it’s important to note that these are not to be slighted, Eowyn is a formidable woman, she is a warrior who wields a sword and bravely charges into battle (although she doesn’t know it yet) and would be capable of defending herself against the onslaught of Grima Wormtongue, who is a weak willed, cowardly and spineless man who wants power for himself, but wants others to do the work for him to achieve it.
But what hope would Eowyn have of keeping her freedom if Gandalf had never freed her uncle Theoden from his torment? The people never would have fled to Helm’s Deep in the first place, so Saruman would have sent an army of orcs to Rohan to slaughter all those who tried to oppose him. So Eowyn either could have been defeated in battle by the numerous orcs, to the point where she was injured or weakened and could be locked away and tortured to the point where she was broken and Grima could claim her, or perhaps she would have been poisoned in mind and soul by Saruman’s terrible powers of persuasion, until she gave in to Grima and his lust.
Either way, Eowyn would have had a terrible life as the slave and ‘reward’ of such a grotesque and vile man. She never would have become the hero that she was meant to be, she never would have met and married Faramir and had their son, and she probably never would have seen her brother again. But with the fellowship’s help, she is one of the best characters in the entire trilogy for many fans.