Elf Tree

Elf Trees (エルフツリー) are trees used by elves for habitation and architecture in their villages.

Background
According to Ainz Ooal Gown, he believes that the magic involving elf trees was developed in the Elf Country in response to the various dangerous monsters that inhabited the Great Forest of Evasha. The ground floor of the forest being inhospitable forced the elves to move to the treetops.

The trees form a central part of elven culture. Elf trees are a variant of trees that are created through the magic of elven druids. Elven druids use their magic to grow and transform trees for habitation and architecture by creating cavities inside for housing. The trees could also be shaped to have bridges between trees. When tens of those structures were bunched up together, they aggregately form an elf village. These trees provided not just shelter and furniture but weapons and armor for the elves. Elven druids use their magic to take care of the trees, such as producing water and cultivating the trees to grow branches to ward away flying monsters and to act as concealment.

Due to the nature of the trees, the entrances to them were located at a height that corresponded to the second of third floors of a human dwelling. On the third floor were the living areas. A typical bedroom of an elf tree is rather primitive, elves sleep on a pile of leaves and use magic to repel insects. While seemingly rough and backward, elves find the bedding to be comfortable and families often sleep together in groups.

From Ainz's perspective this was a type of magic that did not exist in YGGDRASIL. This tree altering magic is apparently only exclusive to elves and only they can use their magic on elf trees. In his hypothesis, the undead believed that the elf trees were perhaps a type of monster that used the elves to multiply themselves. However it has yet to be confirmed by Ainz.

The Half Elf God-kin Arc
and The Half Elf God-kin II Arc Ainz Ooal Gown gained knowledge of elven culture, from the Elven Slaves and was particularly interested in the elf trees. After learning that it was a magic that even Mare Bello Fiore could not do, Ainz wanted to acquire the technique as soon as possible.

Trivia

 * Though the trees are effective at keeping most types of monsters at bay, monster types like snakes and spiders are the natural enemies of elves in the trees. Hence night guards are assigned to keep watch, but still there are casualties as these monsters have high intelligence.
 * The Gigantic Tree was mistakenly assumed to be a type of mutant elf tree, by the Elven Slaves.