The Multiverse, once known as Dominia (not to be confused with Dominaria), is comprised of infinite worlds. is considered the Nexus of the multiverse.
Richard Garfield, in the 1994 Pocket Player's handbook, described Dominia:
Imagine a vast beach. The sand
shifts constantly, moved mostly by the tide and the wind, but also by
the creatures that scurry across it or burrow beneath. Subtler effects,
like compression or changes in temperature, also make their mark.
Sometimes the grains cling together, weathering as a single stone until
they are broken apart by some other force.
Now, imagine that
each of these grains of sand is its own world, and you begin to get a
picture of how Dominia works. Dominia is a multiverse, a collection of
universes. Usually, the inhabitants of a particular world have no
interaction with the other universes; the live out their lives believing
that their home is the “One World.” Even when some cataclysm on a
nearby plane affects the surrounding worlds, the occupants of those
worlds can blame the gods, or perhaps invent nonexistent natural laws to
explain the changes in their plane.
Dominaria is the center of the multiverse. It is connected to 11 other worlds, once known as the shard of the 12 planes, and is one of the largest worlds of the entire multiverse. Imagine it as a large boulder in the center of the beach, with the sands and stones of other worlds shifting around it.
These worlds, and the infinite others in the multiverse, are where our stories begin, to show what planeswalkers can do, and we follow them on their exploration and journeys!
The Known multiverse (2008)
The Planechase sets, both 2009 and 2012 introduced many worlds not seen in various block sets. They are described
here for 2009.
Planeswalkers once were some of the most powerful beings in the multiverse- they were able to create and destroy planes. Some examples of fabricated worlds were Serra's Realm, Mirrodin, and Phyrexia. Planes have also been revealed to have a sort of spirit, a soul, of senses, which can form into a elemental, or speak with animists.
Recent worlds visited:
Ixilan- A lush jungle continent on a plane with the same name, this world is inhabited by Pirates and Dinosaurs, and planeswalkers here have found the inability to leave.
Amonkhet- A world with similarities to ancient Egypt, it is a world ruled by Gods and their trials to greatness for the return of the God-Pharaoh.
Kaladesh- A world of wonder and artifice, where Aether fuels the energy of the inventions created by genius designers. An inventor's fair is held, overseen by the mysterious Tezzeret.
Innistrad- Two blocks have shown the gothic gloomy world of Innistrad, and while the humans of the plane already face werewolves, vampires, and zombies, they now have to contend with the cosmic
horror of the Eldrazi.
Zendikar- A world of intense mana and floating peaks, full of quests and adventures, with danger and temples to explore. It was invaded by the Eldrazi over 5000 years ago, and they have
recently escaped!
Tarkir- A world ruled by 5 dragon lords which was once the battlegrounds of Khans. A time shift has changed it from a world where dragons were once extinct.
Theros- Greek mythology influences Theros, and the gods of the subplane Nyx are the thing of enchantments. Gods here can be invoked, and magical creatures are formed of the living sky. One planeswalker attempted to become a Theros god himself- to what ends, however?
Ravnica- A city world controlled by ten guilds plotting against each other. They are held to law by the living Guildpact, Jace Beleren. However, their world is often visited by planeswalkers, and it is an very unstable peace at the moment. The
past events. The
Recent Events.
New Phyrexia- The artificial world of Mirrodin is no more; the world is now ruled by Phyrexia. This metal world is still forming itself to the new hub of the Phrexian homeworld, to eventually spread further to the multiverse.
Alara- Alara was once broken into five subplanes, each with their own 3 color shards. Bant, Jund, Grixis, Naya, and Esper are now areas within this single plane, but their isolation has caused them to clash.
Fiora- A world of a classical kingdom, Paliano is the hub of
intrigue, culture, and
royalty.
Lorwyn/Shadowmoor- A world alternating between light and shadow, Lorwyn magically changes to Shadowmoor at night through a Aurora, but with inhabitants being able to be in control.
Kamigawa- The Japanese inspired world of Kamigawa is home to a
variety of different races, and has connections to their spirit subplane. A war between the Spirit and Physical world brought choas to the usually peaceful world.
Shandalar- A wandering plane, Shandalar is a world ripe with mana, and very traditional fantasy elements.
Dominaria- The center plane of the multiverse, it is one of the largest worlds. Spanning multiple continents and cultures, Dominaria has a wide variety of races and history- from an Ice Age to multiple world ending events, including time-related catastrophes.
Figured this was a good opportunity to make it as pirate themed as possible. For those that don't know, the world of Ixalan has a organization of pirates, known as the Brazen Coalition, who seek out treasure and riches across the seas. 
