Tsubasa: Reservoir Chronicle

Tsubasa: Reservoir Chronicle is a crossover series by CLAMP. There is an ongoing manga, an original anime series, and several OVAs. It is complimented by xxxHolic.

Plot
Syaoran is a young archaeologist in the country of Clow who is excavating ruins. While visiting him, his childhood friend, the Princess Sakura, reacts to a mysterious symbol within the ruins. Ghostly wings appear on her back and she is drawn into the symbol. Syaoran pulls her out, but the feathers of her wings separate and dissipate into different dimensions. To save her, the priest Yukito sends Sakura and Syaoran to Yuuko, the witch of dimensions, in modern-day Tokyo. There they are joined by the wizard Fay, who is fleeing King Ashura, and Kurogane, a ninja who was banished from an alternate version of Tokyo by Princess Tomoyo. For a price, Yuuko gives them the power to cross dimensions: Mokona Modoki. With Mokona's help, they travel through different worlds seeking out Sakura's feathers.

At the start of the series, it appears rather simple and fun, but after the Acid!Tokyo arc the tone of the manga takes on a much darker tone, and the structure changes radically. In fact, the plot ends being so mindbending that there are chances an agent will go insane even after being exposed to goodfic.

Crossover Elements
Many characters are alternate universe alter-egos of characters from Card Captor Sakura (and the original Sakura also has a part in the plot), notably the characters from Clow. XxxHolic is a complimentary series that has a connected timeline, and Yuuko is a pivotal character.

In addition to this, alternate versions of many characters from CLAMP series appear in different worlds.

So, if in one of CLAMP's work canon is altered, it can have consequences for other CLAMP canons. This means messing with Tsubasa's canon is a Very Bad Idea. Also because it's already a mindscrew as it is.

OFU
The Official Fanfiction University of Tsubasa: Reservoir Chronicle, or OFUTRC, is set in Clow. The canon characters who staff it are taken from all the worlds the main characters pass through.