Remina was once a city where humans and monsters coexisted. It was the only town where humans could leave on Hellgondo because the conditions were less harsh than on the remainder of the continent. Thirty years before the beginning of Luka's adventure, the whole population of the city was massacred.
Humans came to believe that the monsters of Remina had slaughtered its humans, although there were some stories of monster deaths and corpses of both races washed up near Gold Port. In the present day, only chimeras inhabit the area and angel feathers can still be found amongst the ruins.
In reality Goddess Ilias had her angels and Promestein's chimeras slaughter the entire population because their advancement in science were considered blasphemous by the goddess: they intended to use Heinrich's sword to bring the hero back to life, which, if successful, could have revealed some unsavory informations on the actions of the Goddess of Light.
The slaughter had a major consequence: it reignited the conflict between humans and monsters. A few years down the road, the Monster Lord of the time devised a plan to sacrifice herself and make coexistence possible again, but her plan was broken by her daughter who became berserk and viciously lashed at the heroes who had slain her mother.
Most of all, Micaela, the First Seraph, left Ilias when she realized that her goddess had destroyed a whole town to protect her image with the same technology she considered as blasphemous.
The Slaughter of Remina only happened in the true history. In the Paradox World, the death of Ilias in the original timeline made it so she couldn't destroy Remina. As a result, the experiment with Heinrich's sword proceeded as planned, and the Great Disaster happened instead.