In Lenga, there are no real 'undead' so to speak. A thing is alive or it is not. The definition of alive is a union of body and soul. Once the soul has separated from the body, it travels to the afterworld, or it remains in the world of the living as an incorporeal being. The soul cannot travel into another body if it decides to stay in the living world.
The body, on the other hand, can be animated. It can even be given the power to speak and understand simple commands. However, it is not alive. It cannot think for itself and will only do exactly as it is instructed. Animations have no impulse to self-preservation. This means if ordered to march off a cliff, they will march off the cliff without hesitation. The body is just like any other object in this respect - any object (statue, block of wood, pile of dirt) can be animated. A reanimated body is called a skeleton or zombie in other game systems. An animated collection of material is called a golem in other game systems.
Either kind of animation is rumored to be able to understand 1000 words only. There's a story of an ancient golem whose creator wanted to teach the monster every word he could. Unfortunately, he never made it past the A's. Instructions to this golem can only be given in words that start with the letter A. Most magician syndicates have a standard list of 1000 words for 'animations'. A few use Ganarii due to the nature of their spoken language. (Ganarii animations can almost pass as intelligent.)
The soul, however, travels to the afterworld at death. It first travels to Hades. Hades is a holding place for the soul until it decides to travel to the Elysium fields for rest, or to Limbo in search of a way back to the world of the living. Some souls never leave Hades - at first this may be due to indecision, but ultimately as Hades tends to destroy desire, a soul stays because it has no desire to move on.
The Elysium fields are a place of rest for those that choose it. Ultimately, it is a place that allows for continued ambition and desire with the comfort of immortality. The fields are large enough that those that do not wish to be caught up in the ambitions of others can readily escape. As a place that allows for desires, no one can be compelled in the Elysium fields.
Limbo is a place of motion. The soul in Limbo loses its identity. Once 'blank' it returns to the world of the living as another person.
There are gateways to these places in the world of the living. A few even try to direct the incorporeal souls that remain to Hades. Souls that enter directly into the Elysium fields from the world of the living are unique and very rare cases. Most are the 'nobility' of the Elysium fields. Many an emporer has sought to keep their power by attempting to bypass Hades.
However, there are tales of three mages that have chosen a fifth option. These mages were all stricken with the Phage that debilitates some spellcasters. In an attempt to find a cure (instead of seek treatment which robs them of their ability to use magic), these three sought to reconnect the soul with the body immediately after death. When the body dies and the soul separates, the Phage goes away. Seeking to take advantage of this, these three sought to return to their own Phage-free bodies.
To their delight, it worked. To their dismay, however, they found that the body must be sustained by magical power. Since the body died, it cannot produce new blood cells or heal by normal means. These three use magic to repair all damage to their bodies at a very rapid rate. (This extends to the cellular level.) As a result, they never age, never grow hair longer, never shave. Other body systems also shut down, so they do not eat or feel the need for sleep. The only living organ is the brain.
Unfortunately for them, they feel perpetually hungry. Occasionally, they consume the blood of sentient species. Doing so allows them to appear as normal humans in skin color. It also offsets some of the gauntness they are prone to have. One has been driven mad by his condition, the other two may not hold out much longer. They have supposedly recorded their steps in order to duplicate the process.
Only the gods or titans can actually create new life force. It is presumed that the presence of these three has gotten their attention and completely reviled them all. Many believe that the tales of these three mages are simply moral stories to teach children the evils of unbridled ambition.