Catrina, with the help of her servant Jonas, created a potion which allowed her to take on the appearance of the Lady Catrina. She later bluffed her way into the palace using the seal of the House of Tregor. She placed a love charm on Uther and he quickly began bending to her every word. When Merlin began to suspect her, she ordered Jonas to trap him before her marriage to Uther , which went off without a hitch.
Upon realising that Merlin had escaped, Catrina claimed he'd stolen her seal, prompting Uther to order his death. She also made Uther enforce a new tax on the people to satisfy her greed, much to Arthur's dismay. When Arthur released a man who didn't pay the tax, she used this to have Uther believe his son was too weak to rule and have him make her the heir to the throne. However, during her ceremony her troll form began to emerge due to Merlin switching her real potion for a fake. As she tried to escape, she transformed completely, much to the shock of everyone except Uther, who didn't notice any change.
However, her plan ultimately failed when Gaius and Merlin staged Arthur's death, bringing Uther to tears of true remorse. These tears broke the enchantment as he saw Catrina for what she truly was. Despite all attempts to prevent Arthur from drinking the antidote, he recovered from his state of living death and stabbed her resulting in one gassy death (Beauty and the Beast).
Trolls are flatulent creatures whose views on beauty differ greatly from humans'. They have tails, clawed fingers which are larger than that of a human, sharp teeth like big tusks, and are usually grey and pale, with furrowed skin. They also have green blood. They hate fresh food and cleanliness, preferring to eat rotten food and live in filthy conditions. They have an uncontrollable desire for riches, much like goblins. They are rarely seen as they hate all other forms of life. Trolls are also old allies with dragons, for reasons unknown, since their motives are far from similar. Indeed, Kilgharrah even found the idea of Uther marring a troll amusing, implying that the dragons find the trolls to be as disgusting as everyone else. However, Gaius claimed that the dragons were old allies to trolls due to their affinity with magic, implying that the dragons and trolls respected each other's power.
Trolls have enormous strength, able to rip a door off it's hinges and throw a fully grown man across a room. Their breath can turn into green fog making food rot so that it suits their taste.
Trolls have strong magic, beyond most human capacity. They are able to cast powerful love spells, and when someone is under such a spell they will do anything the troll wants them to. Trolls with knowledge of the right spells/potions are able to shape-shift in a way that mirrors can't show their true forms. Indeed, Gaius even claimed that this last is a prove of their immense power. They also seem to have great control over elements, particulary earth, since that troll disguised like Catrina was able to block Merlin's passage with a rockslide. The effort that Merlin, the most powerfull human warlock in Albion, took to remove the rocks from his way futher shows how powerfull troll magic can be.
Though sometimes blinded by greed and their own foul habits, trolls are also quite clever, since Catrina was the mastermind behind such a great plot to take the throne.