Remember when I said, “There are a lot of cool things in Fizban’s Treasury of Dragons. I don’t count gem dragons among them”?
Hollow dragons are cool.
Take a metallic dragon with a responsibility so important that it can’t chance failing to uphold it by dying. Replace its life force with an imperishable aurora of otherworldly radiance—but keep it contained in the former dragon’s metallic hide. Send it back to work.
This new entity is tireless and unwavering, incapable of shirking its duty even if it wanted to. Destroy it, and its parts self-reassemble like the limbs of a troll (at least, the cooler sort of troll). Plus, the visuals, man, the visuals.
OK, so let’s get down to how this thing behaves.
In this case, one thing that I usually save for the end of an analysis belongs up front: Hollow dragons don’t retreat, they don’t surrender, and they don’t let themselves be drawn off task. It doesn’t matter how much damage you deal to them. They do not abandon their posts, full stop.
They have the size, the Armor Class, the movement, the senses, the ability contour and the Legendary Resistance of a typical adult dragon. Their Strength, Constitution and Charisma are all extraordinary, with the first and second of these serving as their primary offensive and defensive abilities. As in life, they remain melee-favoring brutes. In undeath, they gain resistance to necrotic damage and immunity to poison and radiant damage, along with a variety of condition immunities. However, they lose proficiency in Dexterity saving throws, as well as a bit of their former Wisdom (unless they’re former brass dragons, in which case they were only ever slightly above humanoid average in that ability to begin with). Their hit points remain in the same ballpark as before.
The Frightful Presence of a living dragon is replaced with Sapping Presence: There’s something about getting close to a hollow dragon that sucks the energy out of you. Unlike Frightful Presence, a hollow dragon must repeat this action every turn for it to continue to have an effect. Fortunately, it’s part of the hollow dragon’s Multiattack; unfortunately, if the hollow dragon uses its action on Radiant Breath instead, there’ll be no Sapping Presence this turn.
And since it’s on the standard 5–6 recharge, a hollow dragon wants to make use of its Radiant Breath—which deals radiant damage within the standard-issue 60-foot cone—whenever it can, positioning itself where its breath weapon can strike all its foes at once. Otherwise, with its tough natural armor and swift flying speed, a hollow dragon is perfectly happy to chance opportunity attacks from its flyby targets in order to remain airborne and out of melee reach in between turns. There’s never any particular need for a hollow dragon to land, except perhaps to block a passageway it can’t allow its opponents to enter.
A hollow dragon is willing to negotiate in order to avoid a fight it doesn’t have a strong chance of winning, but it has to rely on its natural Charisma; it has no particular proficiency in any social skill. That gives it flexibility in its conversational approach, but not a lot else. However, when combat commences, the hollow dragon is pretty good at assessing its opponents’ weaknesses, once it’s had a chance to observe them in action. In particular, it takes note of which of its opponents aren’t rushing in to try to engage it in melee, presumably because they lack Strength, Constitution or both.
The thing is, a hollow dragon’s damage resistance and immunities don’t help it a lot against front-line fighters—or against arcane spellcasters, for that matter, unless they’re necromancers. They protect most effectively against rogues with poisoned blades, clerics and paladins. Because of this fact, the hollow dragon relies on its metallic carapace and abundance of hit points to withstand direct attacks from both weapons and spells and focuses instead on foes casting spells that call for Dexterity saving throws, its one and only true weakness.
Hollow dragons have three legendary actions: the single-action Claw, the double-action Ghostly Binding and the triple-action Booming Scales. Claw is mainly for use against flying opponents who dare to get in its face, as well as for when its mission keeps it grounded. Ghostly Binding is for use against an especially vexing opponent—generally a spellslinger, skirmisher or marksman relying on an ability other than Strength for its attacks—whom a hollow dragon plans to chastise with an onslaught of follow-up attacks. Booming Scales calls for the hollow dragon to dive down within 10 feet of at least two of its frailer foes before issuing its plangent knell; more would be better, considering the action cost, but the key thing is to make sure that its most problematic enemies are within range.
The relentlessness of hollow dragons is facilitated by their Reconstruction trait, which causes their panoplies to break apart into inert pieces when they’re reduced to 0 hp. Those pieces may no longer pose any immediate threat, but if a hollow dragon’s opponents don’t take the time to destroy them thoroughly, they’ll re-join within a few days—even if scattered around the neighborhood—and the hollow dragon will reanimate, its hit points fully restored.
What it does next depends on whether it still has any chance of fulfilling its original purpose. If that hasn’t been thwarted, the hollow dragon goes straight back to what it was doing. If its foes have defeated its purpose or done something that causes it to no longer make sense, the hollow dragon might shrug and say, “Well, them’s the breaks,” and either find something new to do or allow itself to pass on. Personally, though, I think that in most cases, it should go full revenant, especially if its opponents were total jerks about it.