(Covers information from Anderson's universe)
"I hate being down here. I'm hearing movement, in the walls..."
The first encounter with Majini Undead occured in Citadel Correctional Facility, a prison that had been re-purposed into a shelter by six survivors (Luther, Bennett, Crystal, Angel, Kim and Wendell). As the Majini Undead dug their way into the prison, their presence was felt by Wendell, mentioning to Luther West that he could "hear things in the walls". Shortly after, the Undead broke through into the shower room, going unnoticed for a short while until Alice caught Wendell spying on her in the shower before one attacked him, dragging him away down through the tunnel it emerged from.
When the Undead managed to breach the prison's barricades, some Majini Undeads ambushed Chris Redfield, Crystal, and Alice as the three attempted to swim their way for the armory to obtain weapons and gears inside. One of them managed to ambush Crystal, pulling her back into the water. In spite of Alice and Chris' efforts, the two remaining survivors were soon far outnumbered by the Undead swarm and were forced to flee.
After Alice destroyed a large group of Undead on the roof and dispatched some on the outside of the prison's main enterance with Luther's aid, the remaining survivors (Claire, Chris, Luther, Kim and Alice) attempted to escape the prison through the same shower tunnel the Undead Majini had created earlier. However, Kim was bisected by an Axeman during the escape, killing him and separating the group temporarily. When Alice and Claire were able to reunite with Chris and Luther, the Majini Undead occupying the tunnels attacked them, causing Luther to be thought dead, forcing his group to escape without him. Even so, Luther managed to survive the attack and escaped through one of the storm drains leading to the ocean. He emerges in time to watch as a fleet of Umbrella planes surrounded Arcadia.
At an unknown time, Umbrella Prime managed to recreate mutation that gave birth to Majini Undead in a more controlled environment for their further experiments in creating more powerful bioweapons. The end result of the said experiment was a special parasite that enable them to control Undead and enhancing a living being to superhuman level called Las Plagas. With that parasite, they successfully created a batch of intelligent undead that can use weapons against survivors.
After the destruction of Umbrella Prime Facility, Alice noticed a swarm of Majini Undead beneath the ice, apparently surviving the facility's destruction andmake their way to the surface during her battle with Bad Rain, who had enhanced herself with a Las Plagas parasite. Realizing that Rain was far too powerful to take on in any conventional manner, Alice took aim and shot the ice layer beneath evil Rain, plunging her into the icy depths where the swarm of Undead devoured her alive.
The Majini Undead would later be eradicated, alongside every other T-Virus mutant, following the release of the Anti-Virus into the air.
Externally, Majini Undeads are not different from the standard Undead, yet they seem to possess a pale-greenish coloration, possibly a side-effect of their mutation. However, they possess the ability to extend flower-like tentacles from their mouth to bite down a victim, higher intelligence, and ability to run, giving them more leverage than regular Undead just like Enhanced Undead.
Majini Undeads are proposed to be a product of wild, unforeseen mutation of T-Virus that created them. This is supported by Wesker's statement about how T-Virus enhanced his very being after being administered to revive him, which also endowed him ability to extend flower-like tentacles similar to that of Undead Majinis' (though his version of the tentacles more resembling mandibles full of sharp teeth).
In the draft script for Afterlife, the statement above was hinted Luther and Alice who speculated on the possibility that the T-virus has mutated wildly among the undead populace, allowing them to run, burrow underground, and even swim; the possible mutation in the T-virus is responsible for giving birth to the new undead whom attack their victims with flower-esque mandibles.