Amon

"There is a malevolent presence within the Void that seeks to destroy everything we hold dear. Could this... Fallen One be a Xel'Naga? After so long, could the Gods themselves seek our destruction?"

- Zeratul warning Jim Raynor

Amon, otherwise known as the "Dark Voice" or "Fallen One," is a malignant xel'naga connected to the Void. He is the leader of the hybrids, and seeks to remake all life in its image.

Antiquity
"As I was your beginning - so shall I be... your end."

- The Dark Voice as its hybrids destroy the zerg and protoss

When the xel'naga began uplifting the zerg, they created the Overmind to control and direct the hive mind, so as to try and avoid the "failure" with the protoss. But the Overmind was not given free will. It was given an overriding directive to obliterate the protoss. Amon had a hand in this. Amon saw the zerg as a valuable tool, appreciating their ability to consume essence. He thus bound them to the will of the hive mind, with only the primal zerg escaping his control.

While Amon ultimately planned to eliminate the zerg, it considered the hybrids to be proof of the zerg's success in its greater plan. It intended to enslave the zerg and use them and the hybrids to wipe out the terrans and protoss. Once the hybrids were dominant, it would dispose of the zerg and reshape all life in its own image.

At some point prior to the Great War, Amon ceased to live.

The Countervailing Pawns
The Overmind was not blind to its plight. It foresaw the apocalyptic outcome if the Dark Voice, whom it called "the Fallen One", was victorious. The creation of the Queen of Blades was largely the Overmind's attempt to free the zerg and prevent that future. In the future it glimpsed at, the Dark Voice recognized the threat that Kerrigan posed.

Kerrigan was reborn as the Queen of Blades after Amon's death. Because of this, while she still felt his lingering influence over her, and had her personality twisted, she was effectively an individual, as opposed to the Overmind.

Four years after the Overmind's death, Tassadar, now in spirit form, passed on the creature's vision to Zeratul, who made sure to warn Jim Raynor of Kerrigan's importance and to ensure her survival. Zeratul wondered if the Dark Voice was a xel'naga. Regardless, Raynor followed through, and successfully de-infested Kerrigan on Char through a xel'naga device.

Resurrection
"You've already...lost. Amon...lives. You will...see him...soon."

- Narud's last words.

Kerrigan first became aware of Amon on Zerus, informed of his existence by the primal zerg called Zurvan. She was reborn in primal form, freeing her of Amon's taint.

After returning to the Koprulu Sector, Kerrigan was contacted by a resurrected Alexei Stukov. He pointed her to Skygeirr Platform, which was producing hybrids under the direction of Dr Emil Narud. Kerrigan and her forces fought her way through the station, where she found her way to the xel'naga temple beneath it. There, she battled Narud, revealed as a servant of Amon. Kerrigan defeated the shapeshifter, but in his last moments, Narud revealed that Amon was alive and would be returning soon.

Although Kerrigan nearly died, she was able to glimpse into Narud's mind before he died as well. Initially assuming that Amon was permanently dead, she realized that after killing Mengsk, she would have to face Amon one day, as somehow, Narud was successful in resurrecting him. After defeating Mengsk on Korhal, she departed to do battle with the Fallen One.

Reflecting on her fight with Narud, Stukov speculated that the xel'naga device used to de-infest Kerrigan may have actually had a hand in somehow reviving Amon.

StarCraft II
In StarCraft II, a portrait called "Dark Voice" is available as an achievement reward.

Amon has an in-game unit in the Galaxy Map Editor. The model resembles the voidwalker from Warcraft (although this may simply be a placeholder model), and one of his selection quotes is "I love it when a plan comes together", the signature line of Colonel John "Hannibal" Smith of the A-Team.