Tan (たん Tan?) is the prototype Elixir of Life created by Lord Tensen. By harvesting the Tao of humans from a pit located in Kotaku, Lord Tensen is able to create an elixir that grants them additional lifespans, giving them larger Tao reserves and a form of semi-immortality. Although its magical properties are beneficial, Tan can only be consumed by the Tensens since it will have other consumers undergo Arborification and die.
After they managed to lure in humans to Kotaku and discovered how grand the species Tao was, Lord Tensen extracted their Tao and created Tan that would be a stepping stone to their completion of immortality. For ages, Tan, which was believed to be the Elixir of Life, had been sought out by humans who traveled to Kotaku but fail as they all met their fates encountering the monsters on the island or by Lord Tensen who turned them into Tan.
Despite their success in creating such an elixir, Rien, the leader of Lord Tensen, continued on his research to enhance the quality of Tan by using the Banko in order to create a perfected Elixir of Life.
After realizing the Elixir of Life exists on an island, the shogun sent out five expedition teams to acquire it for himself but was met with failure after the five teams met their fates by Lord Tensen. The shogun then decides to send out a group of death row criminals and executioners in hoping success would be made. The humans that arrived became the first known travelers to have ever made it far in Kotaku and caught the concern of Lord Tensen. Despite the trouble they caused to the group, Rien encountered one human in particular named Aza Chōbē who escaped from the Tan pit and gained regenerative abilities after having his Tao fused with Plant Tao. Rien saw that Chōbē's Tao would be the key to completing his research and persuaded him to follow him to the palace to perform experimentation's. In doing so, Rien managed to create the Elixir of Life that he presented to his creator Xu Fu.