The Hellish Shardzombies, vampires, skeletons, humans, and other creatures. The mages of Grixis hunt for the ultimate powers to conquer all others. They also harvest a strange gas-like material called "Vis," which is how they hang on to Grixis's small traces of life. The undead of Grixis live in the hellish landscape. They live (un-live?) there and rot to death, then rise again. There is no order, yet things are not as wild as in Jund, for without green and with blue, things are more controlled and not completely designed around instinct. The planeswalker of Grixis is the mighty and evil Nicol Bolas, who is the oldest of all planeswalkers. He's one of the Elder Dragons, and is thousands of years old. Bolas is very evil and power-hungry, and he's devised a terrible plan, learning that the Conflux is approaching, and he's trying to inherit the plane's power, even if that means the destruction of the entire plane.
The game part
In Grixis, everything, or almost everything, is undead. Therefore, it makes sense that creatures from Grixis like to rise from the dead. So a lot of cards from Grixis have the keywork Unearth. When a card with unearth comes into play, treat the unearth as if it didn't exist at all. Once that card is in the graveyard, however, the unearth becomes useful.
Say I have in my graveyard and my opponent has no creatures. Even though Dregscape Zombie is in my graveyard, I can pay the unearth cost and actually return in the the battlefield. And it even gets haste! That means that I can immediately attack with the Dregscape Zombie and deal 2 damage to my opponent. When my turn ends, the creature is exiled. So even though you cast use the unearth more than once, it is still good. It lets you overwhelm your opponent until the succumb to your power, because all the creatures that are dying are simply coming back to fight for another turn. If a creature that has been unearthed leaves play, which means that if it dies or is returned to its owner's hand, it is exiled instead of going anywhere else.