Shai'tan

The Dark One is the source of the True Power, which is analogous to the One Power. However, one can only draw on the True Power with the Dark One's blessing. Furthermore, it is much more addictive than either saidin or saidar, and not even the strongest willpower can fight the desire to use it. The True Power is demonstrated to be centered around death and destruction; for example Moridin uses it accidentally to crush the life out of a servant. As of the end of The Gathering Storm, only Moridin and Rand can use the power as they are linked together.
The Dark One has never been actually seen in any of the novels. Padan Fain states in The Fires of Heaven that the Pit of Doom's lake of fire "holds the Great Lord of the Dark in its endless depths." Although this language may be figurative, the Dark One has several powers within the area of Shayol Ghul. He can speak, although his 'voice' is massive enough to inflict incredible ecstasy and pain on the listener, often reducing those to whom he speaks to barely a coherent state.
There are other ways the Dark One can impact the world, as well. Metaphysically, the Bore is a pathway that allows the Dark One to reach out into the physical world; Lews Therin Telamon and the Hundred Companions attempted to seal the Bore to limit the Dark One's power, but over the course of the series his influence extends, and the cuendillar seals on his prison begin to weaken and break. By Lord of Chaos, several seals have broken and the rest are fragile.
As the Bore widens, the following phenomena occur, indicative of the increasing ability of the Dark One to affect the world.

• The Forsaken begin to escape (Aginor and Balthamel first)
• "bubbles of evil", likened by Moiraine to a miasma in a swamp, emanate periodically from the Dark One, twisting reality in the Pattern to lethal effect, especially near ta'veren
• The Dark One often uses the environment against his enemies. In The Eye of the World, he attempted to cause widespread starvation through prolonged winter. The unnatural winter was broken by Rand al'Thor, as he unintentionally uses the store of pure Saidin at the Eye of the World.

From Lord of Chaos onwards, the Dark One visits the burning of extended summer drought upon the land. This freak weather is eventually righted by Nynaeve and Elayne (amongst others) with the Bowl of the Winds in The Path of Daggers. However, by the time of 'Knife of Dreams' the Dark One's influence has stretched so that the food stores of the whole of the Westland are becoming impossibly spoiled, even through a Power wrought warding which is intended to prevent this (and has indeed done so for the last couple of thousand years).

• During The Eye of the World, Rand is convinced the seals on Shayol Ghul are weakening, because 'Ba'alzamon' is showing increasing ability to touch the world. In fact, the seals still held, and Ishamael was simply masquerading as the Dark One.
• Power to resurrect the dead, as with Aran'gar, Osan'gar, Moridin, and Cyndane.
• Most ominously, power to alter reality: in Knife of Dreams, the plans of buildings change randomly, the dead are commonly seen by those who knew them, and saidar itself seems to be weakening. Some characters have voiced the theory that the Pattern itself imprisons the Dark One, and that this seeming unraveling of the Pattern is the last and greatest omen of his emergence.
• Tarmon Gai'don, at which point the Dark One will have his greatest influence.

Speaking the true name of the Dark One, Shai'tan, is believed to bring misfortune on the speaker. In the books, this is represented by the world 'lurching' around the namer and causing shadowspawn to find them easier. Darkfriends claim not to fear this misfortune, but consider speaking his name to be blasphemy. As a result, there are many alternative names for him:

• Ba'alzamon (meaning 'Heart of the Dark' in the Trolloc language, a name assumed by the quasi-bound Ishamael; humans learned it and assumed that it was the Trolloc name for the Dark One, hence the misunderstanding in the first three novels that Ba'alzamon was the Dark One himself)
• Father of Lies
• Sightblinder (used by the Aiel)
• Soulburner
• Lord of the Grave
• Shepherd of the Night
• Heartsbane
• Soulsbane
• Heartfang (used by the wolves)
• Old Grim
• Father of Storms (used by the Atha'an Miere)
• Grassburner
• Leafblighter (used by the Aiel)
• Caisen Hob ("Old Hob", used by the Seanchan)
• Great Lord of the Dark; Great Lord (used only by Darkfriends, the Forsaken, and Myrddraal)

At the time of the books, the Dark One is imprisoned in the same nameless state the Creator placed him in at the moment of creation, but with a "Bore" drilled into that prison. The Bore, made by Mierin Eronaile, was subsequently sealed with a patch made by Lews Therin Telamon and the Hundred Companions at the end of the War of Power. A backlash of power at this moment of sealing led to the taint on saidin, the male half of the One Power.
The Dark One has the most influence in the Pit Of Doom. The Pit Of Doom is an otherworldly place beneath Shayol Gul, a mountain in the valley of Thakan'dar, beyond the Blight. In the books, Shayol Ghul is entirely a real place, but the Pit Of Doom, while it lies beneath Shayol Ghul, is either partially or wholly not in the world. Shayol Ghul was a tropical paradise in the Age of Legends.