Death Oni

The Death Oni was a level 35 miniboss, which could be found in a cave beneath Half Moon Island. It looked like a wither skeleton wielding a sword and helmet. It seemed to be the leader and the most powerful of the other mysterious undead mobs found across the island, including the Oni Zombies, Oni Skeletons and the level 30 Oni Shaman.

On death, the Oni was guaranteed to drop the Rare Level 30 Oni Helm.

Location
The Death Oni resided in the underground tomb below Half Moon Island. A maximum of one instance would be alive at any given time, and it would spawn in the open grave at the far end of the room, guarded by many Oni Zombies and Oni Skeletons. A Tier III Loot Chest could also be found above the grave where the Oni spawns.

In order to access the Death Oni, the barrier in the underground cavern where it resides (on the south side of the island) had to be opened first. Near the top of the ruined tower complex north-east of the cave entrance, where the Oni Shaman could be found, was a pressure plate; dropping any item on this plate would temporarily open the Oni's tomb. The door to leave the tower at the top is one-way only, so the entire ruin must be traversed in order to reach the pressure plate.

Strategy
When the player was spotted by the Death Oni after entering its chamber, it would rush forward to attack immediately. It fought using its sword in close melee combat, so classes with more range had to stay out of its reach, but it had several methods for closing the distance between itself and opponents; its movement speed was much higher than most other mobs, and it also frequently used the Charge and Pull spells to move around the room very quickly or to force the player within its melee range, respectively.

The Oni had a relatively small amount of health for a boss mob, but its maneuverability made it very hard to avoid damage from it if the player did not have access to effective movement spells. In addition, its undead minions spawned in large numbers throughout the chamber, and could quickly overwhelm the player during the fight with the Oni if the player wasn't careful to avoid leaving too many alive at once.

The Death Oni had decent defenses against the and  elements, and it took additional damage from  and  attacks, so equipment based on these elements was very helpful for lower-leveled players.

Trivia

 * The Death Oni, like the other Oni mobs across Half Moon Island, was named after a kind of Japanese demon.
 * The Death Oni used to drop 1-5 Emerald Blocks, but was nerfed due to players making huge amounts of Liquid Emeralds very quickly by farming it.