Cross Gen

A Comic Book company that flourished briefly in the early oughties, then crashed and burned rather spectacularly. Its conceit was a Shared Universe comprised of books in many different genres, all fitting together to create a huge Myth Arc that early Word of God insisted would be at least five years in the making.

Founded sometime in 1998, CrossGen started publishing four comics series (dubbed by fans the "four core" books) in early 2000:

Mystic, a story of magic and mysticism drawn in a conspicuously Art Nouveau style.
Sigil, a Space Opera humans vs. aliens story starring embittered ex-soldiers rediscovering their ideals.
Scion, a sci-fantasy story set in a techno-feudal world.
Meridian, a Coming of Age Story set in a Renaissance-style world with naturally buoyant islands.

These four books featured parallel storylines happening to very different protagonists, set in very different worlds, and aimed at very different demographics. Two mysterious beings (dubbed by fans "Creator and Sidekick") discuss the state of the universe and an upcoming crisis, while the Creator decides to empower individuals on different powers by marking them with the Sigil, which grants them access to the Universal Energies. Each story begins with a hero, the Sigil-Bearer, a thinly-veiled love interest, a mentor with softly-glowing orange eyes, and an antagonist.

Gradually, other titles were added which explored different aspects of the Sigilverse:

The First, the extraordinarily powerful beings who consider themselves the Sigilverse's gods, who discover that these new Sigil-Bearers can siphon off their power and set out to destroy them.
Crux, set on Earth and thereby revealing the Sigilverse to be taking place roughly a hundred thousand years in the future.
Sojourn, a Heroic Fantasy title exploring the idea of an evil, tyrannical Sigil-Bearer.
Ruse, a Victorian-style Detective Drama whose connection to the overarching mystery is...well, mysterious.
Negation, a Mirror Universe that does not obey the rules of physics, where a Ragtag Bunch of Misfits kidnapped from the Sigilverse are chased by the Big Bad of the Myth Arc.
The Path, a Sigil-Bearer story set on a Samurai world.
Way Of The Rat, a Kung-Fu story featuring mythological weapons and described as a "book-end" of The Path.
Route 666, a horror story related to Negation in an initially obscure way.
Brath, a Sigil-Bearer Captain Ersatz of, not Conan the Barbarian, but Robert E. Howard's similar character Bran Mak Morn.
Solus, the title meant to expose the mysteries behind the Sigilverse and reward loyal fans with some concrete answers.
El Cazador and Kiss Kiss Bang Bang, both books set in the Sigilverse's "past."

CrossGen also published a number of books that didn't take place within the Sigilverse, and weren't related to it at all. Since these projects were taken on in the later days of the company's existence, many of them didn't last long, and quite a few were announced but never published, as the company declared chapter eleven and eventually sold all its assets to Disney. Notable among these is Abadazad, a surreal children's fantasy title which was the incentive for Disney buying CrossGen out, and thus continues to be published.

Marvel Comics has announced a CrossGen imprint, starting with 4-issue limited series of Ruse and Sigil. While Ruse still stars detective Simon Archard, the new Sigil series appears to be In Name Only, starring "Samantha Rey, a 16-year-old girl from South Carolina who gets swept up in a war beyond her understanding and may possess the power to turn the tide in the form of a mysterious symbol on her chest even she doesn't understand", and described as "epic fantasy on a colossal scale, ultimately spanning the whole of human history." Subsequently, a third title was added, Mystic, which keeps the basic concept of "magical sisters in a fantasy world and the 'wrong' one is The Chosen One", but relocates it to a Steampunk Disney movie.

Tropes found in the Sigilverse as a whole include:

Call to Adventure: All the Sigil-bearers get a glowy tattoo and strange powers foisted on them without warning. Adventure ensues.
Comes Great Responsibility: With glowy tattoo comes glowy responsibility, whether you want it or not.
Crisis Crossover: The last two comics of the original Sigilverse were Negation War, which was going to consist of all the sigil-powered heroes fighting the villains of the anti-sigil-verse. Alas, the company went under at that point.
Evil Chancellor: Many of the sigil-bearers acquired enemies. Those enemies often then acquired mysterious advisors with strange powers. These advisors were usually members of The First, who didn't trust this whole sigil-giving thing.
Fan Service: The early issues of the core four were rife with it, including PantyShots and Absolute Cleavage. However, a few months in, the creators convinced the senior editor that the books would sell on their innate strengths, and didn't need to pander. You can identify this moment, because all the main female characters suddenly put more clothes on.
Fanwork Ban: Enacted because of Rule 34.
Power Tattoo: The sigil itself, a yellow/red yin/yang-ish symbol that grants great power. Some people get variants, such as the double-tailed orange sigil in Crux, or the negative-space "N" symbol of the Negation.