When a Troll comes to Camelot, it takes the disguise of Lady Catrina of Tregor. She managed to capture the heart of Uther Pendragon by placing an enchantment on a necklace of which she gave to him to make him fall for her.
Merlin and Gaius who were already suspicious, quickly work out that Catrina isn't really Catrina at all but an imposing troll who wants Uther's wealth and power. After marrying Uther, the troll (still posing as Catrina) convinces Uther to disinherit his only son Arthur and have her as the heir to the Throne.
Catrina is then made Queen of Camelot but due to a fake potion that Merlin replaces for her real transformation potion, the potion causes her to transform back into her natural form, much to the shock of Arthur and Morgana. Uther, on the other hand, still under the enchantment, does not seem to care that his 'wife' is actually a troll, since, in reality, he still sees her as Lady Catrina.