The Coronation of Queen Catrina

The Coronation of Queen Catrina took place during the two parter episode Beauty and the Beast which saw King Uther's new 'wife' become the new Queen of Camelot.

Background
When a Troll comes to Camelot, it takes the guise of the Lady Catrina of Tregor. It managed to capture the heart of Camelot's King and it placed an enchantment on a necklace that she gives to him to make him fall him. Merlin and Gaius who are 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 made 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 causes her to transform back into her natural form much to the shock of Arthur and Morgana but Uther, being under the enchantment, doesn't seem to care that his 'wife' is actually a troll.