At the dawn of DC Rebirth, the Green Lantern line was split into two books. Hal Jordan and the Green Lantern Corps followed the eponymous heroes on their space-based adventures. Meanwhile, Green Lanterns followed rookie Lanterns Simon Baz and Jessica Cruz as they grew together as protectors of Earth.
One of their first charges was protecting an exiled Guardian of the Universe, Rami. In the distant past, Rami had created the first Green Lantern rings, to battle the crazed First Lantern, Volthoom. After Volthoom was defeated, the rings were locked away for billions of years because the Guardians feared the power of the Emotional Spectrum. However, Rami wasn't done with rings. He created another ring, more powerful than any other. Based on Volthoom's ring, the First Ring, that provided him with a direct connection to the Emotional Spectrum, Rami created the Phantom Ring.
WHAT IS THE PHANTOM RING?
The Phantom Ring could access the full Emotional Spectrum, and since it directly connected the user to the Spectrum, it required neither a Power Battery nor recharging. What made the Phantom Ring so singularly powerful, however, was that it did not choose a wielder.
Anyone could pick it up and wield the full might of the Emotional Spectrum. However, being mainlined into the full power of those emotions made the ring dangerously unstable, and it would overwhelm almost anyone who put it on. The rest of the Guardians, shocked at Rami's recklessness, sent him into exile, until he found a way to destroy the ring.
WHO IS THE PHANTOM LANTERN
After Rami shows up on Earth with the Phantom Ring in tow, the ring is stolen by Fred Kaminski, a former test pilot obsessed with becoming a Green Lantern. Manipulated by the specter of Volthoom, returned to the land of the living through his connection to the Emotional Spectrum, Kaminski uses the ring to finally fulfill his dream, albeit with a slight catch.
The Phantom Ring is wildly unstable and dooms him to explode. Unfortunately, he's still mainlined into the Emotional Spectrum, which gives Simon and Jessica plenty of trouble in trying to save him. He rattles between channeling all seven colors of the spectrum, based on whatever emotion he happens to be feeling in the moment. Jessica and Simon eventually cause Kaminski to feel compassion, which taps him into the brainwashing Indigo light. Overwhelmed with compassion for those he's hurt in his rampage, he removes the Phantom Ring willingly, which is quickly confiscated by the Green Lanterns. The Guardians lock the Phantom Ring away in a vault on Mogo, where it remains safe for a time.
CYBORG SUPERMAN AND THE PHANTOM RING
The Phantom Ring would remain untouched and unmentioned until the series' final arc. Hank Henshaw, the Cyborg Superman, begins manipulating the Green Lantern Corps by remotely hacking in to the Central Power Battery and corrupting Lantern rings whenever they recharged. His primary target is Simon Baz, who eventually frees Henshaw from his prison in Superman's Fortress of Solitude, and unknowingly hand-delivers the Phantom Ring right to his hand.
Henshaw then uses the ring and a massive armada of Ravagers to assault Mogo. They are pushed back by the remaining Green Lanterns, although they lack the use of their rings until Henshaw is driven away by Hal Jordan, wielding the last uncharged and uncorrupted, ring. Henshaw flees back to Earth, where he tries to destroy Coast City yet again. Hal stops him, and Henshaw escapes along with the Phantom Ring. Oddly, Henshaw never seems to make full use of the Emotional Spectrum while wielding the Phantom Ring, sticking to basic energy blasts and constructs. His escape is also a major dangling plot thread that is yet to be addressed, as the series ended almost immediately after his escape, to be replaced by Grant Morrison's The Green Lantern, which is entirely focused on its own story.