Oni are cousins to ogres, more intelligent, with innate spellcasting ability and capable of regeneration. Unlike, say, trolls, which can be prevented from regenerating by burning them with fire or acid, oni regenerate regardless of what kind of damage they take or how much, short of killing them.
Physically, oni have the typical brute ability contour of very high Strength and Constitution relative to their Dexterity, which is merely average. The fact that they excel at toe-to-toe melee fighting, however, doesn’t necessarily mean that it’s always their first choice. They also have high Intelligence and Charisma, and even their Wisdom is above-average. This means that they can plan, assess their opponents’ weaknesses accurately, use this information to select the targets of their various abilities, and employ deceit as well as raw strength in order to achieve victory. Oni also have proficiency on all of the big three saving throws—Dexerity, Constitution and Wisdom—making them highly resistant to magic, despite not having the Magic Resistance feature per se. And they can fly!
The timing of their abilities may be an issue, though, because despite their various advantages, one thing they lack is any feature that enhances their action economy. Just the opposite, in fact: Their Change Shape feature costs them an action, during which they can do nothing else. On the other hand, being able to cast invisibility at will gives them a way to avoid detection, and by extension damage, while setting up for other things. Oni’s Regeneration makes them masters of attrition fighting: the longer they can drag a battle out, the better. Their opponents have to hit hard and end the battle as fast as possible, or the oni will wear them down.
In addition to invisibility, oni can cast darkness at will. They also have a handful of once-per-day spells. In order of power and effectiveness, from greatest to least, these are cone of cold, gaseous form, sleep and charm person.
- Cone of cold is the big gun, an area-effect combat spell that deals cold damage to every creature in a 60-foot cone, even ones who make their Constitution saving throws. The Dungeon Master’s Guide’s “Targets in Area of Effect” table (page 249) indicates that six is a good number of opponents to nail with this spell. Given that the oni gets only one chance per day to cast it, however, I don’t think it will use cone of cold right out of the gate, even if its potential targets are properly aligned. First, it’s got to have some indication that its opponents aren’t pushovers, and thus that it may need a spell like cone of cold to deal with them. A good sign of this is if the oni’s foes can deal it moderate damage (reduce it to 77 hp or fewer).
- Gaseous form is primarily a time-killer, a way of avoiding serious damage long enough to regenerate if darkness and invisibility aren’t working for some reason. It’s also a way to pursue victims who mistakenly think they can escape by locking a door behind them. One thing it’s not is a particularly effective method of escape, because it reduces the oni’s movement speed to 10 feet, and its enemies can still damage it, just not as much or as fast. It requires concentration, and it prevents the oni from casting any other spell while it’s in effect.
- Sleep ceases to be useful once the squishiest of the oni’s enemies has more than 16 hp—that’s how few an opponent has to have for the oni to have a two-thirds chance or better of dropping him or her. Basically, we’re talking about a wizard or sorcerer of level 3 or below. Not many of these are going to be facing an oni.
- Charm person is useful against a low-wisdom target before combat begins, in order to soften the target up for Deception or keep him or her out of the impending fight. After combat begins, it’s a waste of an action.
As for darkness, since oni have darkvision but not blindsight or truesight, this spell impedes the oni as much as it does its opponents. Therefore, rather than black out the entire area, an oni will prefer to use it to blind its enemies only—in particular, troublesome enemy spellcasters. Darkness requires concentration, so an oni who’s sustaining it can’t cast invisibility without dropping it. Since invisibility is so useful, darkness has to take a backseat unless (a) invisibility isn’t doing the job for some reason, or (b) those enemy spellcasters are really troublesome.
Now let’s take a look at the oni’s other distinguishing feature, Change Shape. There’s some odd wording here that isn’t identified as a typo in the Monster Manual errata: “The oni can polymorph into a Small or Medium humanoid, into a Large giant or back into its true form,” but its true form is a Large giant. I think this has to be interpreted to mean that the oni can polymorph into the form of any other Large giant, which would include ogres, half-ogres, ettins and trolls. (It can’t polymorph into other giants, nor into Large creatures of any other kind.)
Change Shape doesn’t offer any obvious combat advantage except when changing from a Small or Medium form into a Large form so as to increase the damage of the oni’s glaive. But does it offer any non-obvious advantage? No, because the oni’s Change Shape doesn’t alter its stats in any way or grant it a reservoir of hit points as the polymorph spell does; the only things that change are its size and appearance, plus the damage of the glaive, if the weapon is shrunk along with the oni. It doesn’t pick up new features, increase (or decrease) its ability scores or gain alternative attack actions.
So I’m going to conclude that the oni uses Change Shape mainly to pass unnoticed among other people, and that it reverts to its true form only when it’s time to throw down. This will be the first action it takes in combat, but it won’t have much of a chance to catch its opponents by surprise—not if it wants to use its glaive, because if it’s not holding that thing when it transforms, the glaive won’t transform with it. Or, to put it another way, the oni has to choose between catching its opponents by surprise (so that when they finally regain the ability to act, it’s ready to take them on in its true form) or getting to use its preferred weapon at the cost of taking a round’s worth of damage as it transforms.
Well, that’s not exactly true. There’s a third way: First, the oni casts invisibility. Then it retrieves its glaive. Then it polymorphs back into its true form. Then it attacks. The oni doesn’t get a free round to act while its opponents are surprised, but on the flip side, they don’t get free hits on it before it can take a swing at them. I don’t love this as a tactic; it’s not particularly elegant, nor does it have that cinematic quality that the start of a good fight usually has. I think that if the player characters start a fight with an oni (or vice versa) while the oni is passing as a member in good standing of humanoid society, that oni’s got to simply Hulk out there on the spot and fight with just its claws, or with its glaive if and only if the humanoid form it’s using in order to pass unnoticed has a decent reason to be carrying one.
Oni can fly, and thanks to the 10-foot reach of their glaives, the hover-in-the-air, fly-down-to-attack, fly-back-up tactic that many other monsters use is feasible even though they can’t easily avoid or shrug off opportunity attacks. On the other hand, being a brute rather than a skirmisher, an oni may be content simply to fight toe-to-toe. You can play it either way.
How does an oni choose whom to attack? It depends on how badly its opponents are able to hurt it. If they’re all (or mostly) doing comparable damage, it will want to reduce their numbers, and it will focus on taking weaker opponents out of the fight right away. But if one or two opponents are doing far more damage than everyone else, then it’s not going to worry about the weaker opponents; it’s going to focus on the strongest. And, obviously, it’s not going to attack anyone who’s under the sway of its own charm person spell.
Oni won’t willingly initiate combat during the daytime. If an oni chooses to attack, it will do so at night, when it has the advantage of darkvision. In most cases, though, oni aren’t interested in starting a fight at all, at least not one with opponents who could give them any trouble. What they want is to be able to kidnap people and eat them in peace. A fight with a group of experienced adventurers puts that whole state of affairs in jeopardy. On the other hand, sometimes oni join forces with other evil creatures, and in these instances, they may fight on those other creatures’ behalf; this is when PCs are most likely to get into a drawn-out battle with one or more oni.
If an oni has no interest in standing its ground, it will cast invisibility as soon as it’s moderately wounded and never show itself again. An oni that’s serious about defeating its foes, on the other hand, will fight the aforementioned battle of attrition, taking advantage of its Regeneration ability to outlast its foes. After any round in which an oni’s opponents deal it more than, say, 30 hp of damage (enough to bring it down in five or six combat rounds, taking its Regeneration into consideration), the oni will cast invisibility and lie low the following round, plus an additional round for every 10 hp of damage its opponents did above and beyond that 30 hp threshold. Then it will position itself behind its chosen target and attack, becoming visible again as it does. (It doesn’t gain advantage simply by attacking from behind—that’s not how flanking works in fifth-edition Dungeons and Dragons, even if you’re using that optional rule. It’s just cooler this way. If you could turn invisible and sneak up on your enemies to attack whem, why wouldn’t you do it from behind?)
Alternatively, upon being moderately wounded (reduced to 77 hp or fewer, after Regenerating), if it can position itself to catch all of its opponents—or six of them, whichever is fewer—in a cone of cold, it will cast that spell rather than take its Multiattack action. If its opponents include a non-elf spellcaster with 16 hp or fewer, it will drop a sleep spell where it can catch that opponent and as many others as it can, though if it knocks anyone out aside from that one target, it will be merely icing on the cake. It will use gaseous form only to pursue its enemies or to gain time if they can penetrate its invisibility (remember that, while the oni can cast invisibility at will, it can cast gaseous form only once).
Even an oni that wants to win a fight will recognize when it’s seriously wounded (reduced to 44 hp or fewer, even after Regenerating) that it’s not going to end well, unless its last opponent is just barely holding on. In that instance, it may cast and sustain invisibility just long enough to Regenerate back above 44 hp, then try to deal the coup de grâce to its opponent. Otherwise, it will simply use invisibility and its flying movement to flee the scene. If an opponent is able to penetrate its invisibility, it will use the Dodge action as it flees—and if it manages to Regenerate back up above 77 hp, it may decide to turn around and start fighting again!
Next: I answer readers’ questions.