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

The Dark Voice (also known as the Fallen One) is a Void entity. It is the leader of the hybrids. Its goals include the eradication of the terrans, protoss, and zerg.

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

- The Dark Voice as its hybrids destroy the zerg

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. The Dark Voice had a hand in this.

While the Dark Voice 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.

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. The Dark Voice recognized the threat that Kerrigan posed.

After the Overmind's death, its vision was passed to Dark Prelate Zeratul through high templar Tassadar. Zeratul 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.