The Makaiju (Hell Tree) itself is an alien tree that nourishes Ail and Ann, but has become unhealthy and requires energy to stay alive. For some time they supply it with human energy, but eventually it stops working. In the dubbed version and ADV subtitles, it's referred to as the Doom Tree.

When the aliens try to give Usagi to it as an offering (as it seems to greatly like her energy), it becomes angered and starts to injure those around it, killing Ann in the process. It did not stop until its alien children realize love. The tree tells its story to Sailor Moon, saying that it once had many children to whom it would give energy, but eventually they became greedy and began to fight over the power. (The English dub, however, changed the tale by claiming that "evil forces" were responsible for corrupting its children.) The full story was that the "Doom Tree" was originally known as the Tree of Light, which lived alone on a faraway planet on an island in a vast ocean for countless years, so it decided to create life in the form of some humanoids. In the end, there were only two left, two small children with whom the tree fled into space. It was now weak and needed the energy of love to survive. Sailor Moon uses her power to purify the tree, and it disappears. When Ail and Ann reunite, a small sapling appears to them; the tree has been reborn and they are given a chance to start over, and they leave Earth forever for parts unknown.

(Source: The Oracle :: BSSM Encyclopaedia)