Titania

Queen Titania is an Archfey of the Feywild, and the Queen of the Summer Court.

Background
Queen Titania was considered a powerful and influential archfey whose powers were able to bend her surroundings to her will.

She used to set traps in the Feywild for outsiders. Once, a powerful balor named Naviask entered the plane to destroy it, and fell in one of those traps. Titania's magic changed the demon's essence and objective, turning him into a good fey spirit with the mission of "healing scarred lands with nature."

Trivia

 * Outside of Critical Role canon, Titania is considered both the queen of the Summer Court and the ruler of the Seelie fey. However, in Exandria she is solely mentioned as queen of the Summer Court, while Lady Elmenore the Unforgiving is the Archfey holding the title of "Matriarch of the Seelie Court".
 * Outside of Critical Role canon, she is usually portrayed as having a romantic yet complicated relationship with a character named Oberon: they are considered lovers in 5th Edition rules, and husband and wife in older editions and in A Midsummer Night's Dream. Oberon has been mentioned as the kind of character who would be an archfey, but hasn't been confirmed as a character in the Critical Role universe.
 * James Haeck, co-writer of Explorer's Guide to Wildemount, stated that Oberon's presence in Exandria's Feywild is strongly implied by Titania's own presence in the book, although he also stressed that the proper confirmation should be said aloud or put in print.