Jeska was a warrior in Otaria, and the sister of Kamahl. She was trained by Balthor, like her brother. However, she had studied with the dwarven clans behind Balthor's back and was more of a dwarf at heart than Balthor ever was.
The dementia summoner Braids corrupted Jeska using the Mirari, and transformed her into the abomination Phage. Phage in turn fused with Akroma to become Karona. Ultimately, Karona was destroyed. At that point, Jeska's latent planeswalker spark ignited, saving her from destruction.
Centuries later, during Dominaria's temporal crisis, Jeska helped the other planeswalkers repair Dominaria. She fell under the influence of Leshrac. She defeated Radha, and took her to help repair the rifts.
Enraged by the disappearance of her friend Karn and manipulated by the evil planeswalker Leshrac, Jeska used Radha as a kind of living conduit to close the rift in Zhalfir. Radha’s spark wasn’t consumed, but some of her lifeforce was. Radha was nearly killed. The phased-out section of Zhalfir was lost forever, unable to phase back in.
Later, Multani used his maro-sorcery to partially stabilize the rift in Yamivaya. But Jeska, still under the influence of Leshrac, forcibly closed the rift, once again using Radha as a resonant mana-channel. When the weakened Multani tried to stop her, Jeska destroyed him. Multani was lost, dispersed within Yavimaya. Radha lost her potential to become a planeswalker.
When Leshrac was destroyed by Nicol Bolas, Jeska was at last free of Leshrac’s control. Tortured with guilt and pain over her losses and her actions, she accepted that she must be the one to close the rift in Otaria where Karona was felled. She entered the rift and disintegrated. Jeska died, but her last act triggered the Mending, the chain reaction that closed all remaining planar rifts.
"My brother and I both come from Balthor's forge. Kamahl has a temper of fire. I have a temper of steel." (Jeska)
As Phage drank their energy, a vague memory of Jeska stirred. Then she lost herself again in the joy of her victims' suffering.
"Her lips suck forth; see, where it flies" -Christopher Marlowe, The Tragical History of Doctor Faustus (Syphon Soul)
"Even the threat of power has power." -Jeska, warrior adept
"Pained defiance or shameful surrender -every reaction is a pleasant surprise." -Tibalt (Browbeat)