Terra

Terra was a Life Essence created by the Alchemist Kierkan. She wanted a more independent life, and escaped from Kierkan's lair in order to possess one of The Charmed Ones. She possessed a businessman and forced him to lead her to the sisters. She gained a temporary victory when she possessed Piper Halliwell. She also possessed two more people, a Businessman and Leo, Piper's fiance at the time. Although her own fight to have a life, led to her death, The sisters vanquished her after finding out that she had possessed Piper all along.

Escaping Kierkan's Lair
Kierkan takes Terra's desire for independence for ingratitude, since he created her out of his blood. Terra thinks he sees her as just his masterpiece. Just as Kierkan is about to suck Terra back into her essence bearer, Terra stabs herself with a knife--the only way she knew to leave a body--and floats away

Possessing The Businessman
Believing that possessing a Charmed One was the fastest route to greater power, she possessed a businessman in the street. His soul was now only seen by Terra and only seen through a mirror. She told him to bring her to the Charmed Ones. At the manor, the man begged Terra to get out of his body, as that he was starting to suffocate. Terra curtly refused, pointing out that the suffocating feeling was because his soul was dying.

Possessing Piper Halliwell
While at her 10 year reunion at P3, Piper took out the trash. Terra, still in the businessman's body, followed her out and stabbed herself with a knife. As Piper bent down to help, Terra possessed her.

Back in the club, Piper communicated with Terra via a mirror, since Terra was the only one to actually see her soul. After standing up to one of Piper's classmates, Piper summoned enough strength to warn Prue that something was wrong. Just as Piper was about to explain, Terra regained control, acted like nothing happened and went back to the manor. Meanwhile, Kierkan found the businessman's body and brought him back to life long enough to find out where Piper/Terra went.

Terra tried to get Piper to tell her what her powers were, knowing that Kierkan was after her. Piper refused unless Terra left her body. Terra called P3 and told Prue that there was a demon in the manor. Prue suggested that Piper freeze the demon until she and Phoebe could get back. Terra was visibly intrigued. Kierkan then broke into the manor and knocked Terra down. He intended to suck Terra back into her essence bearer. Before Kierkan could finish her off, however, Terra froze him. Prue and Phoebe then came home and they chanted a Power of Three spell to vanquish Kierkan. As Kierkan went up in flames, he told Prue and Phoebe that he could have helped them--but now "she" is about to destroy them.

The sisters went to the attic, suspecting that a female demon was after them--not knowing that the "demon" was in Piper's body. As Terra left the attic, Piper once again gained control over her body and voice and weakly told Prue that she was dying, but Terra made up another excuse to get out of the room. By this time, Terra was determined to stay in Piper's body, knowing that Piper's soul was starting to burn out. When Piper begged her to leave, Terra reminded her that the only way she could do so is with a knife to the heart. Piper saw Leo orbing into their room, and told Terra that her boyfriend "Tom" would know something was amiss. When Leo kissed Terra, he noticed that her kisses weren't the same. Terra blasted him with an Energy Beam, knocking him out. Terra then went to P3 and started seductively dancing on the bar, giving guys drinks as she went along.

By this time, Prue and Phoebe realized that Piper was possessed, and rushed to the manor. They tried to get Terra out of Piper with a spell they wrote, but the spell required the Power of Three. Terra reminded them that since she was created from Kierkan's blood, she had an alchemist's powers--making her equivalent in strength to an upper-level demon. She tried to head back to the party, but Prue flung Terra into the dumpster. Terra went back inside and Prue knocked her out by knocking her over the back of the head. They brought her back to the manor, and Prue flung a knife into her stomach, releasing Terra's essence.

Possessing Leo Wyatt
After Prue stabs Piper/Terra, Leo rushes over to her aid. At the same time, Terra's essence floats out of her body and possesses Leo, who was leaning over her. This was actually part of Prue's plan; she figured that since Terra has an alchemist's powers, she can bring Piper back to life. Prue offers to let Terra keep Leo's body if she brings Piper back to life; otherwise, she goes back in the essence bearer. Terra agreed and resurrected Piper. She then destroyed the essence bearer. The sisters then used the spell Prue and Phoebe wrote earlier to eject Terra from Leo. Terra realizes she's been double-crossed and tries to blast the sisters, but Prue deflects the blast into a mirror. Terra was vanquished almost immediately, as she'd destroyed her only means of survival.

Possession
Possession is the ability to control living beings actions by entering their body. Terra had used this power constantly throughout the episode. She first used this power to posses a Businessman, whilst possessed, Terra was able to communicate with the human's soul, which was slowly dying, the longer Terra stayed in the particular body. After possessing Piper Halliwell, she temporary gained her power of Molecular Immobilization. She also possessed Leo Wyatt and temporary gained the powers of a Whitelighter, including Orbing.

A life essence is toxic to the host body, since it can't handle having two souls in it. The people Terra possessed suffocated from the inside out, and had they not been removed, they would have died leaving Terra in them.

Energy Beam
This power allows the user to shoot deadly beams of energy at whomever they wish. It's usually used with his/her's hands. Terra, used this power at Leo who found out that she had possessed Piper, after Leo notices her kisses are different. She also used this power to attack Prue and Phoebe Halliwell, but Prue used her power to deflect it, making it crash into a piece of furniture. Since Kierkan created Terra, all of her powers come from him, so he possessed this power, as-well as the others she used.

Resurrection
This particular power is very rare. It's the ability to resurrect yourself and others after dying or being killed, either by normal or supernatural means. Terra had used this power, to bring Piper back to life, after Prue flinged a knife into her stomach. This allow Terra's essence to be freed, but accidentally possessed Leo who was standing over her. Terra, as Leo then used Resurrection to bring power back to life. They then said a Power of Three spell to expel her spirit from Leo.

Molecular Immobilization
After possessing Piper Halliwell, she gained her Warren Power of Molecular Immobilization. This allows to user to slow down the molecules of an object to the point where they stop moving completely, therefore freezing the object. After calling Prue to find out Piper's powers. Kierkan burst through the doors, Terra fell back on the floor and told him that he just pissed off A Charmed One, Terra does the stance Piper does when she usually uses this power. The freeze then wore off when Prue and Phoebe come and they chant the spell.

Whitelighter Powers
Although Terra never actually used these powers, she did possess them briefly. After Prue stabbed Piper, Terra's soul possessed Leo, and with her obtained all of his powers. Before she could actually use any of these powers. The sisters use a spell to eject her soul out of Leo's Body, and set him free. He was then able to heal Piper who hadn't been fulled healed when Terra brought her back to life.

Power of Three
Since Terra knew that the only way to actually kill Kierkan was the Power of Three. The Power of Three is a term that involves of three beings, in this case, Prue, Piper and Phoebe Halliwell. After possessing the middle sister, who has the power of Molecular Immobilization, she was able to trick her sisters to vanquish Kierkan. Although, before fully getting vanquished, she says to Phoebe and Prue "I could've helped you. but now . . . she'll destroy you" hinting them that something isn't right with Piper.