Bosses and classes

Any of 28 mob types can come out as a boss. This page follows a boss from the moment its mob spawns to the moment it leaves.

Which mobs turn into bosses

bosses.yml lists them: zombie, skeleton, piglin, pillager, spider, enderman, wither skeleton, vindicator, stray, creeper, husk, witch, drowned, evoker, phantom, vex, zombie villager, bogged, blaze, breeze, cave spider, piglin brute, hoglin, ravager, guardian, zombified piglin (grown ones only, adults_only), the Ender Dragon, and the Wither. Each type has its own spawn_chance (a zombie’s is 9%), and enabled: false keeps a type ordinary.

How a mob becomes a boss

1. A mob of a listed type spawns. Spawn reasons in spawn.allowed_spawn_reasons count: natural spawns, spawners, trial spawners, eggs, raids, patrols, and a few more. Mobs a structure places as the world generates, like a bastion’s brutes or a mansion’s illagers, count as CHUNK_GEN when they first enter the world.
2. It waits until a player within 32 blocks could see it (spawn.on_sight). So bosses come where players are, not in caves nobody walks into. With on_sight off, the mob rolls as it spawns, with a player within spawn.spawn_radius.
3. It rolls once in its life: the type’s spawn_chance, times the multipliers for the world, the time of day, and the weather (world_multipliers, time_multipliers, weather_multipliers under spawn). Walk off and come back, and it stays what it was. A mob with a name, an owner, or a leash never rolls.
4. If it turns, a puff of smoke marks it. It picks a class, takes a level, and gets its health, damage, gear, and name.

Three things raise the chance:

• A full moon. On a full-moon night in a world with a sky, bosses come 1.5 times as often, and the rarest class twice as often (full_moon). Players get a message as the moon rises.
• The survival bonus. As in Sacred, the longer a player plays without dying, the more bosses find them: +25% after an hour, +50% after three, +75% after nine, toward double. Death takes it all, and idle minutes don’t count (survival_bonus). /mobosses info shows yours.
• Weather and night. Thunder ×1.3, rain ×1.1, day ×0.7. The Nether ×1.4 and the End ×1.6.

Two things keep bosses from piling up, around mob farms and spawner rooms too:

• After a boss spawns for a group, its players wait about a minute for the next one (spawn.player_cooldown_seconds), across relogs and restarts.
• No boss spawns while three are already within 48 blocks (spawn.max_nearby, spawn.nearby_radius).

The Ender Dragon and the Wither

Both roll like any mob, 25% before the multipliers, and neither cooldowns nor crowds stop them: each is a fight someone set up. A built wither rolls through BUILD_WITHER in spawn.allowed_spawn_reasons. The dragon rolls whatever the list says, but never in a world’s first fight: only once it’s been beaten and someone brings it back with end crystals.

As bosses they keep the game’s own health bar, in its own color, with the boss’s name, class, and level on it, in locales.default. They never despawn and never step out to reach you. Abilities that would move them, pull you up to them, copy them, or build around them stay off: teleport strike, grapple, chain, doubles, obelisk, fire ring, and web ring. The dragon summons phantoms and the wither wither skeletons, a pack for each player they fight, set on that player: phantoms over their head, skeletons beside them; and enrage’s Strength adds to their attacks. The dragon’s blows and breath grow with its level as its fireballs do, and it drops its loot at each player’s feet, since it dies over the exit portal. Their experience stays near the game’s own up to about level 30 and tops out like a level 100 Mythic’s.

Classes

A new boss picks its class by weight. Each class multiplies the boss’s stats and unlocks abilities at its own levels. Rarer classes come less often.

Class	Weight	Health	Damage	Experience	Color
Common	63	×1.0	×1.0	×1.5	white
Uncommon	28	×1.3	×1.1	×2.0	yellow
Rare	9	×1.6	×1.3	×3.0	aqua
Epic	3	×2.2	×1.7	×4.0	purple
Legendary	2	×3.0	×2.2	×6.0	gold
Mythic	1	×4.0	×2.8	×8.0	red

Classes also multiply armor (defense) and toughness. All of it lives in classes.yml. A boss of Uncommon or rarer glows in its class color while a player is within 32 blocks (effects.glow).

Level

A boss takes the average level of the players within 24 blocks (spawn.group_radius), so one veteran doesn’t drag newcomers into level 90 fights. Now and then it comes a little weaker, by 5% to 30% of that level (progression.variance), so weaker bosses keep turning up. Player levels explains where players’ levels come from.

Stats

Each stat is the mob’s own value, times the class multiplier, times a curve by level. The curves climb fast at first and level off toward a ceiling (formula in config.yml), so a level 100 boss is hard without being absurd. Class differences shrink at high levels.

• Health. A zombie boss has more health than the Ender Dragon (200) from level 30 as a Common and from 20 as a Rare. It passes the Warden (500) from level 40 as an Epic, 20 as a Mythic, and 100 as a Common.
• Damage. Hits grow toward the Warden’s at the top. Arrows, fireballs, and tridents grow too, by a share of it (formula.ranged_damage).
• Creepers. The blast widens with the level: 3 to 8 blocks, or 6 to 11 when charged. A creeper that blows itself up gives nobody anything.
• Never weaker than vanilla. A level 1 boss has at least its mob’s health, damage, and armor.

./gradlew balance prints these curves for your own config; see Building from source.

Difficulty

The game's difficulty grades each new boss: a world's own on Paper, the server's on the mods. A boss keeps what it was made with, so changing the difficulty moves only the bosses that come after.

Difficulty	Modifier
Peaceful	no bosses
Easy	0.7333333337
Normal	0.8777777773
Hard	1.0, the balance as built
Hardcore	1.2333333339

Each part of a boss takes its weight of the modifier: 1 + (modifier − 1) × weight. Health and damage take all of it, so a normal boss has about 88% of a hard one’s health. Armor, minions per summon, and the spawn chance take half; cooldowns divide by half of it, so a hardcore boss casts about 12% more often. The vanilla experience it drops takes all of it, and Mo’Bosses experience follows its health. Loot, rewards, and levels stay the same on every difficulty. A boss is never weaker than its vanilla mob on any of them.

difficulty in config.yml holds the modifiers and the weights. enabled: false plays every difficulty but peaceful as hard, as before 2.11.

Gear

bosses.yml gives each type weapons and armor by level: a level 5 zombie swings a wooden sword, and a level 60 one may carry enchanted netherite. A boss takes a random piece within 7 levels of the best it qualifies for. It never swaps the gear its mob spawned with for a weaker piece of the same kind.

In the fight

• Boss bar. Hitting a boss shows you its health bar, until you walk 64 blocks away (boss_bar).
• Names. Each player reads the name over a boss in their own language (name_tags.per_player); see Languages.
• Announcements. A new Epic boss or rarer tells the players within 64 blocks (broadcast.spawns_from).
• Groups. A boss grows by 35% for each player past the first who truly fights it, up to six (group_health). Someone who only stands by changes nothing.
• Death. Dying to a boss burns your progress toward the next level, never the level itself (progression.death_penalty).
• Whom it chases. Each player’s damage counts as threat, fading by half every 6 seconds. A hit from a player whose threat beats that of the boss’s target by a quarter draws the boss to them, so a vanilla creeper no longer chases its first target while someone else hits it harder (aggro in config.yml).
• No tricks. A boss never gets into a boat or a minecart and never goes through a portal. Hit it from where it can’t reach you, from a pillar, over a wall, or while it sits in a hole or under a trapdoor, and it steps out next to you: after 8 seconds when it can’t find a way to you, after 6 when it can’t move a block. When nothing next to you fits it, as on top of a pillar, it pulls you down instead, with slow falling. It does this once in 20 seconds at most. Within 3 blocks of it you’re in its reach and nothing counts; an archer that can see you just keeps shooting; flyers and guardians never step out. anti_cheese in config.yml holds the switches and the seconds. Damage from lava, falls, or cramming already earns nobody anything, since only players’ and their pets’ damage makes a share.

When a boss leaves

A boss nobody comes within 64 blocks of for 10 minutes leaves without loot, as vanilla mobs do (despawn). Bosses from /mobosses spawn, the Ender Dragon, and the Wither stay.

Bosses keep their type, class, level, cooldowns, and who hit them how hard on the entity itself. They survive restarts and chunk unloads, and pick up their glow, bar, names, and abilities again when their chunk loads.

Next: Abilities