Yggdrasil was a boss in The Great Cave Offensive: Revisited, and makes a cameo as a boss in the 2009 New Year's Tournament. While his gender is unknown, he is often described as a male regardless. His figure resembles that of a large hulking golem, with a very distinguishable difference - his left side (your right) is composed of a darkish purple substance and is often called his magic half, while the other half is made of metal. In the center part where the halves join, a hexagon shaped plate is placed on its upper body, with six red crystals near each vertex of the hexagon and one more larger one in the center. It is believed that this plate is a sort of magical artifact that holds Yggdrasil's two halves together.
Yggdrasil was the result of a group of mad scientists who wanted to determine whether or not affixing a man-made product to pure magical energy was possible. While the machine half was constructed, hunters were sent out to track down a source of magic. Having captured Mizu, the hunters returned and used Mizu's power to generate Yggdrasil's magical half. However, Yggdrasil proved unstable and the halves would fall apart until sealed together with an unknown magic artifact.
Once Yggdrasil was born, he decided to demonstrate his power by destroying the lab in which he was created. From that point his whereabouts were largely unknown, however it would not be wrong to link the recent destruction of the random villages nearby the location of the lab to him. He was eventually recaptured by some of the lab workers that managed to escape and ordered to recapture Mizu. As a result, Yggdrasil was sent underground and literally absorbed Mizu into his body to make himself more powerful than before. After losing, however, he was supposedly destroyed, but because his magical half is very powerful, it isn't actually known if he's dead for good or not.
Yggdrasil's body is almost impossible to destroy with brute force, as seen when a large portion of Loren's attacks to the body fail to do a thing. In addition, even if his metal half is damaged, he can shed the armor, then use his magical abilities to repair it. Because his magical half is composed of raw magical energy, it is virtually indestructable. In addition, his offensive ability is off the charts as well. His machine foot causes shock waves and fissures when he stomps, and getting stepped on is almost guaranteed to be the end of you. He is capable of firing magic missiles, which, while slow, explode into sparks when they make contact with something. He can also fire drills with a similar purpose, however, they are a bit faster than the magic missiles but do not explode.
Yggdrasil is not invincible, however. The artifact on his body that holds together the two halves of his body seems to be the only thing that keeps him stable. Without it, Yggdrasil's halves crumple. In order to destroy it, one must break the six crystals around the central crystal first, which weakens the central crystal as this crystal draws power from the other six. Once the seventh is destroyed, Yggdrasil falls apart. However, be warned as his magic half is raw energy and may run wild once it's no longer contained by the artifact.