Oni


 * Source: Monster Manuel page 239
 * Type: Raider, Spellcaster, Stealth

Introduction
The oni is loaded with abilities that allow it to sneak up on unsuspecting party members and gain surprise. Most notably, the oni can cast invisibility at will, allowing it to sneak in and out of combat easily. Combined with it's fly speed it's almost impossible to catch an oni once it decides to bail. In addition, the oni has passive regeneration, which allows it to attack again, completely healed. The oni can also cast darkness at will, but it is important to note the oni cannot see in the magical darkness. Once per day the oni gets the following spells: charm person, cone of cold, gaseous form, and sleep. The oni has a low spell save DC for its level, so it's save spells are less useful but still powerful. The oni's attacks do fairly weak damage for its level, but the oni is hard to kill so they can add up. The oni's final ability is change shape, which allows it to polymorph into any small or medium humanoid, or a large giant. This can be used to gain surprise, disguising as a trusted friend or a weak begger. The oni is capable of sowing discord by useing charm person while in the form of someone else. In all, the oni should use hit and run tactics and mess with the PCs heads.

An oni has above average intelligence, wisdom, and charisma, and can apply advanced tactics to their attacks. They attack if they feel threatened, or just for fun.

Attacking at night is preferred for an oni, hopefully in a place of their choosing. Multiple escape routes are a must, so outdoors, or near something suitable to being passed through while gaseous is ideal. The oni will use invisibility to stall while flying around.

Running an Oni Encounter
An oni attacks under the cover of darkness unless it is surprised in broad daylight. Oni's are cowardly creatures and thus will attack only with surprise, unless the group in weakened or the oni is caught off guard. The two methods the oni has are at-will invisibility and the change shape ability. An oni might attack alone, or arrange an ambush with underlings.

The oni has a powerful spell in cone of cold that can hit the entire party if unprepared. After that, the oni begins a series of hit and run attacks by attacking for a few rounds then escaping with invisibility to heal up. The oni will target the weakest perceived creature but will not risk undue opportunity attacks. While the fly speed of the oni is slow, it is easily capable of flying out of the range of melee fighters.

All sorts of allies can be attracted to an ogre mage. It might fight allongside other giants, using their brute force to attack the front line while the oni gets up close and personal with the casters. An oni might also get support from a hag or evil wizard, allowing it to take on more of a fighter role than an assassin. Regardless, the oni feels no compassion for its allies and will leave if things look bad.

To beat an oni, the party either has to nova it before it can run away or shut down all of its escape options. The best option the oni has is a combination of invisibility and flying, and any grappling or grappling spell will pin down the oni for an easy win. In a large fight an oni can be incredibly difficult to pin down and kill, and should be saved for last. The easiest way to beat an oni, thought, is to demoralize it. If the party proves the be far stronger than the oni (or got lucky on their rolls), the oni might steer clear of them.