They were originally the normal people who were infected with Cracher-Mortel venon, causing them to that turn into zombies. In this state there not only aggressive, but in wild violence. The antidote is the only solution to cure a people from zombism, which can turn them back to normal. But if its not administered in time they die.
In this state there durability and strength was greatly increased.
Cracher-Mortel have been infrecting people for centuries. In this state the vicitm turns aggressive, then they appear dead, only to wake up some time later. Now they are overcome with aggression destroying everything they see and wildly attacking people. They are also now under the control of the Crachel-Mortel that infected them and will obey there every comand. Crachel-Mortel commonly use them as slaves and to start revolutions.
In 1700's a Cracher-Mortel known as Dutty Boukman, used his powers to infect many slaves on the plantations, and then used them to spark a rebellion. He was followed and watched by a Grimm who recorded his exports, as a warning to future Grimms. It was Cracher-Mortel like him that were responsible for inspiring the belief in voodoo.
In the Modern day Baron Samedi arived in Portland, under orders from Eric Renard. There he began infecting and kidnapping people as part of a plan to cause chaos. One night he released some of his zombies and sent to Portland, as well as leaving clues for Nick to follow. The Zombies were apprehended by the police, and forced under intense sedation.
Tracking the baron, with the anitdote made, Nick, Monroe, Rosalee and Juliette arrived. Seeing them, Baron Samedi started to release the zombies to go after them. Nick and Monroe managed to defeat a few, and cure them. But overwellmed by the numbers they were forced to run. Nick spotting the baron, chased after him. But was infected himself and captured. This being the true plan, to make the Grimm Eric Renards. The Baron took Nick to the private plane to leave for Austria.
Meanwhile Sean Renard was attacked by Two Zombies, having been following his brother. Using his hexinbiest abilities he managed to overpower them. While Monroe, Rosalee and Julliette were forced to run to higher ground. The zombies in close persuite. Fortunetly Hank arrived with the entire police force who managed to overpower them and place them in one of the strongage containers.
Meanwhile the Baron was relaxing on the plane, when Nick woke up. Being a Grimm he reacted differently to poision. Despite becoming a Zombie, his Grimm powers were significantly enhanced, making him near unstopable, plus the Baron could not control him. He caused the plane to crash, killing the Baron but just walked away from the wreckage. Delerious and feral.
Julliette and Rosallee managed to make enough anidote and converted it into a gas. Exposing it to the victims they managed to cure the Zombies. Meanwhile in his zombified form Nick arrived at a roadside bar. Originally confused and delious, a man tried to push past him. Setting him off, he attacked everyone. With enhanced strength, durabilty and reflexes, he managed to mop the floor with all the tenants that didn't run. One man drew a knife at him, but Nick causally over powered him and beat him hard. (That man later died of his wounds)
Escaping Nick, later arrived outside a home where a family was going in. Furiously he chased them and broke in trying to kill them. Monroe and Hank caught up and tried to stop him, but Nick was to powerful. They managed to get him to chase them into the barn. There they temporarily imprisoned him. Captain Renard arrived. And the three of them tried to take Nick on. But he was still to powerful for all of them. Foretunetly during the fight, Julliette managed to injected the anidote into Nick. He managed to make a full recovery, however the implications of his actions while a Zombie were serious, and being a Grimm the venom changed his body, leaving a lasting affect, the full extent of which has not yet been revealed.