When we first came across Théoden he looked a man so old and feeble you’d wonder if he could even leave Meduseld. After a little chat with Gandalf we see him riding off to battle. What happened? And how came he to be dotard in the first place?
Gríma Wormtongue. This cunning servant of Saruman must have had a part of his master’s power. “You are old,” he must have said to Théoden once, “and you’re not getting any stronger.” It may have been true, perhaps, but this is all the more a testament to the subtlety with which Gríma presented the thought. OK, so maybe Théoden may have been drugged, but I think the greater evil was done when Théoden accepted these words with its deeply-hidden lie.
But Gandalf comes. He breaks the spell decisively (but probably not completely, alas!) with a combination of hope:
[Gandalf: ]Not all is dark.
LR III 6
. . . and the truth:
[Gandalf: ]Nor does your age lie so heavily on your shoulders as some would have you think.
ibid
However, Gríma will not give up without a fight. He tries to replant the lie:
[Gríma: ]I care for you and yours as best I may. But do not weary yourself, or tax too heavily your strength.
ibid
Then he tries to denounce the bringer of truth:
[Gríma: ]Dear lord! It is as I feared. This wizard has bewitched you.
ibid
But all his attempts failed:
[Théoden: ]If this is bewitchment, it seems to me more wholesome than your whisperings. Your leechcraft ere long would have had me walking on all fours like a beast.
ibid
And then the serpent is revealed for what it really is, and all its plots laid bare:
[Gandalf: ]Down, snake! Down on your belly! How long is it since Saruman bought you? What was the promised price? When all the men were dead, you were to pick your share of the treasure, and take the woman you desire?
ibid
Ah, Mighty Whitey. What an entrance you make. As my sister had said waaay above, light has indeed come into Meduseld, and all deeds of darkness are exposed in it.