The Undead are an evil group of creatures that are once Living creatures but become undead after death. Undead legions are sometimes formed when various alive creatures are sealed in cocoons and then Skeleton warriors emerges from their cocoons making them have no skins or any organic body parts just bones and dark armor and capes. Darth Grievous and the Corrupted Shockwave are known to have powers to control the undead. Darth Grievous used skeleton creatures to capture Shockwave and web him up to transfom him into a demon general that has power over undead and skeleton creatures. Megatron also has the power to control the undead by infusing himself with Dark Energon but his death made him a dark energon ghost. After the Transformers Wars the undead stayed dead and never risen again.
In the Shattered glass universe Optimus Prime with the power of Dark Energon can control the undead. Dirge was one of the first victims of Dark energon and latter cybertron the dead world of the Transformers was now a zombie planet due to optimus prime threw dark energon to revive all the dead creeps off of the planet.