Flameskulls weren’t among the original undead creatures of Advanced Dungeons and Dragons; they existed only in supplementary material until the fourth edition of D&D, when they first appeared in the Monster Manual. I can’t help thinking of them as being comical and cartoony (It’s a skull! That’s on fire! And hovering! And talking to you!), but in fact they can be a dangerous foe, especially to low-level characters.
Being undead, they have to be saddled with some sort of compulsion. Following the fifth-edition MM flavor text, their compulsion is obedience—specifically, to their duty of protecting a place, item or person. Because they’re bound to this duty, they have no self-preservation impulse; if they must fight, they fight till they’re destroyed.
And as it happens, it’s tough to destroy a flameskull: its Rejuvenation feature causes it to re-form, fully healed, one hour after being reduced to 0 hp, unless its fragments are sterilized with holy water or a dispel magic or remove curse spell. So an unhappy party of adventurers may vanquish a flameskull in order to entire a forbidden area, only to find that they have to fight it again on their way out. (Of course, this raises the question of whether a flameskull that’s sworn to keep them out of a place has any duty to keep them from leaving that place once they’ve already been in it. Play that one as it lies, dungeon master.)
Other features of the flameskull include resistance or immunity to various types of damage and debilitating conditions, magic resistance, spellcasting, and both darkvision and Illumination. This last combination is curious: it can shed light up to 30 feet away, but it can also see in the dark up to 60 feet away. Why would it need both? It doesn’t. It’s an undead creature, therefore non-evolved, so its abilities don’t necessarily have to make sense. But to make what sense we can of it: Darkvision is what the flameskull relies on; Illumination is simply a spooky special effect. I’d say that the flameskull’s base state is dim glow, and that it escalates to bright glow either to let trespassers know they’ve been spotted or when it gets its dander up. Either way, these features don’t have much bearing on the flameskull’s combat tactics, except that it doesn’t use darkness to hide.
The flameskull’s ability scores are almost irrelevant, because it has nothing but ranged, magical attacks. The one that does have bearing on its behavior is its Intelligence of 16, because this means it can make smart targeting decisions. Being undead, it knows to prioritize targets who can drive it away from its task (clerics, paladins), along with those who can match it in magical ability (wizards, warlocks, sorcerers, druids).
Its base attack is Fire Ray, which it can use twice per Attack action. It has +5 to hit with this ability, doing 3d6 fire damage with each hit. Let’s assume that the average adventurer has an armor class of 15. That means that the flameskull has a 55 percent chance to hit, and therefore the total expected damage of its Fire Ray Multiattack action is 12 hp. (This is assuming, however, that the targets are more than 5 feet away. Fire Ray is a ranged attack, meaning the flameskull has disadvantage on its attack roll if it tries to use it at point-blank range. That reduces its chance to hit to 30.25 percent and its total expected damage to just 6 hp.)
Now let’s look at its spells:
- Fireball does up to 8d6 instantaneous fire damage to every target in a 20-foot-radius sphere. Going by the area-effect estimates on page 249 of the Dungeon Master’s Guide, we can expect this to catch four targets. Figure that the average adventurer’s Dexterity saving throw modifier is +3; this means that a player character has a 55 percent chance to dodge, and therefore the expected damage is 22 hp per target, or 87 hp altogether.
- Flaming sphere does 2d6 fire damage to creatures within 5 feet of it; however, it does this on the targets’ turns, and most intelligent beings, feeling the toasty warmth of a 5-foot ball of fire, will feel the urge to move away from it. Happily, however, flaming sphere is a sustained spell that adds bonus actions to the flameskull’s action economy—specifically, ramming it into enemies. This also requires a Dexterity save, so the expected damage is 5 hp per target per turn, over up to 10 turns. (In my experience, however, combat encounters rarely last that long—usually they’re over within four or five rounds, sometimes in as few as two.)
- Blur is also a sustained spell, so it can’t be cast at the same time as flaming sphere. What would make a flameskull want to cast blur instead? Perhaps it’s been hit by a weapon or ranged attack that dealt one of the types of damage it’s neither immune nor resistant to: bludgeoning, slashing, acid, radiant or thunder.
- Magic missile fires three darts of magical force that always hit and that do 3.5 hp of damage apiece, on average, for a total of 10 hp expected damage—less than a pair of Fire Ray attacks. Cast using a 2nd-level spell slot, it fires four darts for a total expected damage of 14 hp, and there’s a good chance that the flameskull will have a 2nd-level slot available, since it can’t sustain both blur and flaming sphere at the same time.
- Shield is a reaction and the no-brainerest of no-brainers in the magical arsenal. Anytime an opponent rolls between 13 and 17 on an attack, the flameskull pops shield.
- Mage hand is just so that the flameskull can play solitaire when there are no adventurers to scare off.
Scenario: Adventurers show up. Flameskull says, “You! Shall not! Pass!” Adventurers say, “Oh, yeah, we shall pass. We’re passing.” DM says, “Roll for initiative.”
First, do the PCs have more than 40 feet of room to move around in? If so, the flameskull knows that they may spread out beyond the range of its fireball spell, so it will cast this spell before they have a chance to. But if not, the flameskull will keep this spell in reserve, at least for a round, just in case it’s not needed.
If one or more PCs act first, and they deal bludgeoning, slashing, acid, radiant or thunder damage that the flameskull can’t block with shield, its first action will be to cast blur. If they don’t deal any of these types of damage, or if the flameskull is able to block it, it casts flaming sphere instead, and it rams that ball of fire into any PC displaying a holy symbol—clerics first, then paladins—or, barring that, into anyone who looks like a spellcaster. The flameskull also casts flaming sphere if it gets to act first, because of the bonus actions it will provide.
On subsequent rounds, when the flameskull has taken moderate damage (reduced to 28 hp or fewer), it decides playtime’s over and drops fireball on the PCs (action). If it’s cast flaming sphere, but it’s also taken at least 12 hp of bludgeoning, slashing, acid, radiant or thunder damage, it rams the flaming sphere into a PC one last time (bonus action), then drops flaming sphere and switches to blur (action). Otherwise, it aims Fire Ray attacks (action) at any PCs not within melee range: clerics first, then paladins, then other spellcasters, then anyone else. If all its enemies are engaging it in melee, it casts magic missile at the highest available level, distributing the darts among clerics, paladins and other spellcasters until they’re all downed. Whatever action the flameskull takes, it rams its flaming sphere (if it’s sustaining this spell) into a PC as a bonus action, and from the second round on, it prioritizes PCs who are engaging it in melee (clerics first, then paladins, then other spellcasters, then anyone else).
The flameskull doesn’t retreat or flee, ever, and once it’s run out of slots for a spell it would otherwise cast, it simply substitutes the Fire Ray Multiattack action. If it’s ever turned, as soon as the turning wears off, it beelines right back to its post and resumes its duties, attacking anyone present if they deserve it.