Theradras

Princess Theradras was an enormous earth elemental and the only daughter of Therazane the Stonemother. After mating with Zaetar, son of Cenarius, Theradras became the mother of the first centaur.

Birth of the centaur
Though most of the elementals were sequestered in the Elemental Plane by the titan-forged, Theradras, daughter of Therazane, evaded capture and hid below the earth, eventually falling into a long slumber. Many millennia later—1,100 years before the First War—tauren shaman accidentally awakened Theradras, having mistaken her faint whisperings for their voice of their benevolent deity, the Earth Mother. The years of slumber had weakened Theradras' form, and she reached out to the verdant lands around her, draining their life energies to restore her power. In that moment, the fertile grassland of Mashan'she was transformed into a barren desert which would later be renamed "Desolace" by the horrified tauren.

The monumental loss of life sent ripples throughout Azeroth and even the Emerald Dream. Zaetar, the first son of Cenarius, emerged from the Dream to investigate and eventually discovered Theradras in heir lair. However, instead of imprisoning the elemental, he became enticed by her stolen life energies and fell in love with her. Theradras realized the influence she had over the half-stag being, and—as she found Zaetar beautiful as well—claimed that together the two could restore the land's beauty. From their forbidden union, the centaur race was born. The first centaur were allegedly Gelk, Kolk, Magra, Maraudos, and Veng, who became the first five khans. The centaur saw the disgust in their father's eyes and brutally killed Zaetar, but Theradras harshly chastised them for their actions. Realizing that they had broken their beloved mother's heart, the centaur begged for forgiveness and promised to honor Zaetar's memory. Theradras entombed Zaetar's spirit in her lair, which the centaur would later name Maraudon and treat as holy ground. The elemental princess herself would sequester with her dead lover's body in her secret sanctuary, the Crystal Caverns of Terramok.

World of Warcraft
Theradras loved the centaur, and she was worried that the Horde's rise to power might eventually drive them to extinction. After years of hostilities with the orcs and their allies, she acted to save her progeny. She infused the centaur with her power, driving them to make war and claim new territories outside Desolace. Most centaur were deeply loyal to Theradras, yet the Magram clan had no such loyalty and thus were perfect allies for the Horde. The Horde helped suppress the other, violent tribes and then its champions turned their attention to Maraudon itself. They journeyed into the depths of the burial ground and slaughtered every creature inside it, including Theradras. Their victory halted the spreading corruption in Desolace and stopped the violent bloodlust in the centaur. With the Horde's blessing, the Magram became the leaders of the united centaur and led them to an age of tranquility. Yet none knew how long they would stay on the path of peace.

Cataclysm
Theradras returned as one of four lieutenants during the Elemental Unrest aiding Cho'gall and his Twilight's Hammer. According to the dialogue with the ogre, it appears that Terramok served as Theradras' prison. She no longer viewed it as such because of Zaetar's remains that are buried there, and she refused to leave. She was then defeated once more by the armies of Garrosh Hellscream and Magni Bronzebeard.

Though she is already deceased in the lore, she can still be killed and her death is a requirement in an updated Princess Theradras (quest).

Her mother Therazane was angry at the mortals for killing her only daughter.

After Theradras' ultimate destruction, a new elemental leader emerged. He wanted to recreate the centaur and led them to reclaim the world from those who had defiled it.

Trivia

 * Before her debut in World of Warcraft, her story was detailed in the now non-canon Warcraft: The Roleplaying Game.

Speculation
The reason why she did not respawn in Deepholm like other earth elementals after their death could be because she was never linked to the Elemental Plane in the first place since she evaded capture.