Obsidian B12 - A chemical compound created using many substances to be capable of tapping into space-time rifts.
Rift Totem - A magical item crafted specifically to activate Obsidian B12.
By making a portal frame with Obsidian B12 and igniting it with a Rift Totem, a Worldgate will be created. Go through it to enter the Ulterlands.

Paramount Woods

A lush, beautiful forest, rich in both flora and dangerous fauna. It's overgrown with roots, grass, trees and flowers. Occasionally, you'll see an ice floe or mossy rock. 

Crimson Cinders

A vast expanse of rock, molten magma and pillars of stone. It's a stuffy place with only flames and glowing stones as light sources. Geysers spew ash from the ground in random streams. Few deep-rooted mushrooms struggle to survive.
Edge Wastelands

An unending, empty landscape decorated only by small patches of purple stalks and dry bushes. The land is cold and the stone is dry. Floating rocks dot the sky.
Cosmic Plane

The Ulterlands are separated from our known world by a fragile veil. By breaking through it, the player has caused bits of deep space to manifest. The area is a mishappen landscape of curious matter, unorthodox plants, broken constructs and bright lights.
Quagmic Slopes
An earthen landscape made of many minerals and stones. Pillars of layered rock stand erect atop the ground. Small shrubs grow from bits of fertile soil wedged between the cracks of the dry surface.
Parapax Realm
A nearly monochromatic realm existing parallel to the Ulterlands that is inaccessible by regular means. It is a dark, cold place filled with vile, powerful monsters that will test your strength, determination and skill to see if you're worthy of the power they can provide.

There are many mobs vaguely similar to those we know such as...
Combustle (Creeper), Ent (Iron Golem), Mushogg (Pig),
Pyre (Blaze), Trodder (Strider), Osteon (Skeleton),
Fortross (Shulker), Endertaker (Enderman).

Some are simply unique...
Starshot,
Gyroc, Aegin,
Tentacule, Riftborn,
Oriath, Oriath Queen,

Bazzernaut, Dodonzo, Flyvern.

And there are even new bosses to summon...
The Perished, a cruel variant of the Wither.
and The Edgerider, a demigod from a time long past.

The Aether Arts are a unique subset of magic that relies on two items:
An Aether Wand capable of outputting raw but weak magic and an Aether Orb that defines the traits/effects of the magic displayed.

Incredible feats can be performed with the unfortunate price of life force.
Ordinarily, the wand will drain your XP as a power source but if you have an insufficient amount (less than 1 level) it will begin to consume your health.

Aether Orb - Divine Rage

When in the off-hand slot,
All magic bolts from the Aether Wand will summon twin lightning strikes (if contact with an entity is made).

Aether Orb - Divine Grace

When in the off-hand slot,
All magic bolts from the Aether Wand will heal 10HP (if contact with an entity is made).

Aether Orb - Divine Curses

When in the off-hand slot,
All magic bolts from the Aether Wand will apply the effects: Blindness, Weakness and Slowness (if contact with an entity is made).

Aether Orb: Divine Summons

When in the off-hand slot,
All magic bolts from the Aether Wand will spawn 2 Osteons to protect you (from the ground where the bolt lands). 

Aether Orb: Divine Radiance

When in the off-hand slot,
All magic bolts from the Aether Wand will cause the target to float upwards and apply the effect "Inferno" (if contact with an entity is made).

In the Ulterlands, there's an alternate version of Redstone that can be used to make 4 new Redstone Blocks!

Trick Bomb - Similar to TNT except there is no fuse timer. The instant it is powered, it explodes.
Stripbore Block - A special block that digs straight down in a 1x1 line and collects blocks for the player.
Lift/Drop Blocks - These two blocks can teleport a player up/down when powered (so long as they Shift Right-Click)