Phased bosses aren’t an everyday food. If every bandit leader and his brother has multiple forms, the whole concept loses its impact. But for key battles, the sort that accompany campaign milestones or end arcs, there’s little more satisfying than watching the PCs win a fight they know they can’t handle. Because phased fights bring deadly challenges down to a reasonable level by manipulating encounter pacing, they fit snugly between “possible” and “impossible” in the exact place memorable fights live.
Designing a phased boss gives you plenty of opportunity to flex your creative muscle. But before you design anything you have to decide what sort of fight you want. There are different types of phased encounters, though the types themselves are means to an end. What really matters is the impression you want to give the players, and that impression affects what type of encounter works best.
Chaos: multiple similar enemies. Many enemies attack the party, but they are all below the party’s level. They enter the fight in waves either at predetermined times or when the current enemies hit a certain threshold. Examples: A kobold worker raises an alarm, summoning small groups of guards every few rounds as they rush from the places they occupied. A horde of zombies storm a house, breaking through boarded-up doors and windows as the fight progresses.
This is appropriate for swarm encounters, where you want the PCs to cut down enemy after enemy until they’re left standing on a pile of bodies. Each individual monster is essentially worthless, but en masse they can easily overwhelm a party’s bad tactics or poor judgment. The exact level of each enemy is irrelevant as long as they can affect the combat; don’t send fifteen monsters who can only hit on a natural twenty, because then all you’re doing is making the PCs waste their time. Each creature should be simple, with straightforward attacks and a minimum of bookkeeping, and I believe they shouldn’t even have limited-use abilities unless that use is obvious. Neither you nor the players want to keep track of which duergars have already used their invisibility power, but it’s easy to see which ones have used enlarge. Your goal is to act quickly, playing up the fast-changing nature of the fight by keeping the turn order moving.
Dread: multiple similar enemies, with one capstone. Many enemies attack the party, but they’re mostly fodder. The final wave is a proper boss, entering the fight once the party is weakened by minions. Examples: A soldier orders her underlings to attack the party, then her lieutenants, until she deigns to face them herself. Orcs attack a dwarven outpost, distracting it until their war machine can make it around the mountain pass and knock down the walls.
The first part of these battles uses the same advice as the swarm above. But the early phases here have a specific goal: wearing down the party so the final phase can wipe them out. Even if the final phase is just an ordinary person, anything looks intimidating when the party is already beaten and bloodied. Pick monsters that support or foreshadow what the last phase is going to do. Your goal is to play up that last wave and make the party feel every hit they take, every spell they cast, every resource they won’t have when the big gun finally arrives.
Progress: multiple distinct enemies. A single boss attacks the party. At some time another boss attacks, and further bosses attack until the fight is over. Examples: The party must work their way through a gauntlet of elementals to prove themselves to a reclusive summoner. Several assassins are moving through the castle, and each must be killed before they make their way to the king’s chambers.
Every phase counts, but not to the party. They count to the enemy. With each phase they beat, the PCs whittle down the enemy’s forces until nothing is left standing. This works best when the players know how many phases there are so they can budget accordingly, even if they don’t know exactly what each phase does so they can’t prepare for it. It’s best if every phase is unique in some way. Fighting three gargoyles in a row isn’t fun, but if each gargoyle has a different monstrous form and different powers, that’s much better. Your goal is to make each fight feel like an accomplishment as the party drives the enemy into a corner and deals a finishing blow.
Triumph: one enemy, multiple forms. A single boss attacks the party. When a certain trigger condition occurs, it changes into a different boss. Examples: A cultist of a snake god becomes a naga when the fight turns against him, and when he takes further damage the spell transforms him beyond his control. A warlord enters battle wielding a sword and shield, switching to his terrain-destroying greatsword only when he gets desperate.
Even more than in other types of fights, it is critical that each phase is meaningfully linked. It’s ridiculous for your vampire to suddenly change into a demon no matter what Castlevania says. The players should be able to see that they’re working through the boss’ tricks, forcing him to adopt new strategies or use new equipment or enter his final form and use 100% of his true power. Each change should follow logically from the ones before it, and ideally each should be stronger than the last as the PCs back the boss into a corner. Your goal is to have both sides ramping up together until the final climactic moment when both the party and the boss throw everything they have at each other and the party emerges victorious.
Obviously there’s wiggle room here. You can makes the players dread an enemy with multiple forms, and one of my favorite phased fights involved multiple distinct enemies who worked by making the battle as chaotic as possible. But I feel the easiest route to each impression is through these paths.
In all cases, no phase should be as difficult as a single reasonable encounter. D&D expects that the party will have some sort of rest time between encounters, even if it’s not a proper short rest. You cannot throw three normal encounters back-to-back at the average group and expect them to be fine. You have to scale every fight down based on how many phases your battle has. In my experience, a phased battle can be about 40% more difficult than a normal battle per phase beyond the first:
|Number of Phases||Total XP||XP per Phase|
That is, if your party can normally handle 400-XP encounters, and you want a boss with three phases, the total XP budget should be about 720 XP and each phase should be about 240 XP, around the level of an easy encounter. You can take some of your budget from one phase and add it to another, but your total budget should stay along these lines, and no one phase should be more difficult than a normal encounter. Also, these numbers assume that the phased boss is a large part of a short day. If it’s the day’s only encounter, you can scale the XP budget up slightly, and if the PCs have multiple fights before or after it, definitely scale it down.
How much downtime you want to leave between phases is up to you. An archetypal phased boss has each phase follow immediately after the last, but you could give the party a round or two to heal up, refresh their buffs, or get into position. You can present this as a boss’s transformation sequence, or a brief monologue, or the time it takes to rush from one scene to the next, as long as it gives the players a chance to breathe. You can also have phases overlap, either intentionally (the hill giant appears when there’s only one ogre left alive) or otherwise (the hill giant appears on round 5, and you really thought the PCs would be done with the ogres by then, but no). I don’t have modifiers to the above numbers based on the amount of downtime, so treat it as a course correction. If the party is struggling, increase the time between phases, and if they’re knocking it out of the park, speed things up. Even a single enemy with multiple forms can use some power or ability from their next phase before the actual transition. Imagine the players’ faces as the demon lord hurls a fire lance when his hit points get low, and then after he transforms that fire lance is a power he can use every round.
If you’re new to phased bosses, I suggest starting small. Use easier encounters than recommended above and see how your PCs handle it. Some groups are better than others at pacing themselves for marathon fights. When in doubt, make it clear how many phases a boss has or give the party some way to learn it through skill checks, deduction, or storyline. If the party hears through the grapevine that the fey queen has different powers based on the seasons and they see paintings of all four forms decorating the walls of her castle, they might be surprised when they slay the spring queen and the summer queen rises to continue the battle, but they don’t get to be mad when the fall queen follows. Foreshadowing isn’t just for narrative, it’s also a way to help the players prepare for challenges.
You’ll get a better and better feel for how to make phased bosses over time. I got started on phased fights in 4E, where any boss who doesn’t get stronger at half health is objectively wrong. The bloodied mechanic is the simplest form of phased battles, one that translates into 5E with almost no effort. More complicated phases, like those discussed above, take more work to create and balance, but the effort is worth it for truly climactic battles.