The Massacre of the Zehoberei was a mass murder carried out by Thanos in his quest to bring balance to the universe.
Millennia ago, after exhausting their finite resources to deal with overpopulation, the eponymous inhabitants of the planet Titan struggled to come up with a solution. When the outcast Thanos proposed massacring half the population at random, thus allowing the other half to thrive, the people deemed his proposal far too extreme and dismissed him.
Eventually, the Titans, save for Thanos, were exterminated by the cataclysm they feared would happen and the planet was rendered unrecognizable. Convinced his plan could have saved his people, the guilt-ridden Thanos set out on a violent campaign to prevent other planets from experiencing Titan's fate.
Thanos' ideology, along with his growing power and vast armada of both Chitauri and Outriders, cemented his reputation as a feared warlord, further convincing him that the only way to bring balance to the universe was to exterminate half of all life. He began seeking out planets suffering from overpopulation and overwhelming their defenses before massacring half the populace. Eventually, his quest led him to the planet Zen-Whoberi, which was on the brink of collapse.
Leading the Black Order and a contingent of Chitauri soldiers, Thanos successfully invaded Zen-Whoberi and rounded up the members of its populace. Ebony Maw then ordered that the population be divided evenly into two groups, one that would be spared and another that would be executed.
In the midst of the chaos, a young girl named Gamora tried to fight off the Chitauri soldier restraining her so she could find her mother. Thanos, who witnessed this act, took her aside and explained his beliefs through the use of a switchblade that he balanced on his finger. He then asked Gamora to do the same just as Maw ordered the Chitauri soldiers to open fire on the second group.
As the soldiers carried out their orders, Gamora started to turn around only for Thanos to bring her attention back to the switchblade and compliment her when she managed to balance it just as he did.
Following the massacre, Thanos believed that the surviving Zehoberei had overcome the collective trauma and would thrive. However, by 2014, Gamora was known to be the last remaining member of the Zehoberei.
Gamora, meanwhile, was raised to serve as one of Thanos' enforcers. Unlike her adoptive siblings, Thanos grew to genuinely care for her and she became his favored daughter while he built her up as "the deadliest woman in the galaxy."
When she discovered what Thanos had done to her home planet, Gamora began resenting her adoptive father's actions and eventually betrayed him, putting her training to good use as a Guardian of the Galaxy.
- In the comics, the Zen-Whoberi were completely annihilated in a possible future by the Universal Church of Truth when the race declined the offer of joining the cosmic cult. Gamora was saved by Thanos during the massacre and brought back in the past, when her race was still alive.
- Gamora mentions in Guardians of the Galaxy that Thanos executed her parents in front of her and that she is the last remaining member of her species; however, Avengers: Infinity War shows that she was on her back seeing Thanos' knife when he gave the order to the Chitauri while Thanos states that half of her species is still thriving.