Zeal is the Queen of the Kingdom of Zeal in Chrono Trigger. She channels Lavos' power from deep within the earth by using the Mammon Machine. Near the very end of the game, only her face and hands appear during the battle aboard the Black Omen. She is the "boss" enemy on the Black Omen.
While the Mammon Machine is channeling Lavos' energy, Crono stabs it with the Ruby Knife. Energy is discharged and they all find themselves on a different dimensional plane where Lavos finally emerges. The monstrous Lavos easily defeats Crono's party and leaves them within an inch of their lives.
It's then that the Prophet reveals himself to be Magus in disguise. Magus then attacks Lavos but is also defeated.
Lavos then tries to steal the life force from Crono's party, Magus and Schala, while Zeal stands back and laughs. Mustering some strength, Crono tries another attack, but Lavos obliterates him, leaving no remains.
The party, Schala and Magus find themselves transported back to the Ocean Palace...without Crono. Zeal remained with Lavos.
After Crono's party makes the long journey through the Black Omen, they confront Zeal. When Zeal is pretty much defeated, she throws them into the Mammon Machine where the party battles this Machine in a whole other dimension.
When they defeat the Mammon Machine, the party then has to fight Zeal once again. All that's present is a magical image of her face and hands. After Zeal is finally defeated once and for all, Lavos appears...which leads the party into the final battle.