Big Chief

Bosses

A good place to start researches is the famous page of Tommi Gustafson.
Some minor nits there.

what's a boss?

Difficult. The term itself is a bit vaguely used. You meet some monsters mightier than others of the same kind, and you certainly notice some ranking. Configuration data also is a bit foggy for determining the circumstances of being a boss. We should change perspective here.

ingame aspects

Now, that's easy. At first.
You'll meet monsters with a golden name of their own displayed on their life bar. Prominently the five Act Bosses, special Quest Bosses, and more or less randomly spawned Unique Bosses with some minions. Further a normal monster can become a boss monster while it is in vicinity of a just activated Monster Shine, it'll get an golden unique name of its own then. This one won't get minions of its own, unlikely to map-generated bosses.
All these bosses are immune to AI-bending effects. Their minions, recognizeable by 'Minion' displayed on their life bar, will inherit some abilities.
Those Minions show a slightly different behaviour than their common comrades. They're subject to AI-bending effects, but nevertheless will attack those who are attacked by their boss. They also guard their boss, you can't lure them successfully far away without the boss. The distance is something about 20 yards, a bit more than a sreen's size in 800x600 resolution.

technical aspects

There are quite a few places in configuration data, where bosses are marked.
In MonStats.txt every non-player unit of the game is listed. A column of its own flags units as 'boss' here.
This covers Act Bosses

Andariel
Duriel
Mephisto
Diablo
Baal Crab
Baal Crab to Stairs
Baal Crab Clone

Baal Crab to Stairs is by the way the non-attackable Baal in Throne Room.
Further units are specified as boss:

Bloodraven
Radament
Summoner
Izual
Nihlathak

This are some Quest Bosses without assignment of a common monster class. Yeah, I know, Radament looks like a Greater Mummy, fights like a Greater Mummy and dies like a Greater Mummy. Still he has his own entry, maybe in dawn of the game he had another graphic appearance.
Surprisingly another complete monster species is defined as boss also.

Putrid Defiler1
Putrid Defiler2
Putrid Defiler3
Putrid Defiler4
Putrid Defiler5

Another file defines the fixed boss monsters which spawn in every game at the same location. They're called Superuniques, and in fact SuperUniques.txt is the configuration file. Each of those bosses is based on a common monster class, including the Countess and the Cow King. Also some entries are unused here.

Further the game randomly generates unique bosses (and Champions) via Rarity and umon{1...10} in Normal respectively nmon{1...10} in higher difficulties depending on Levels.txt.
Those are generated when the map is drawn, unlike to the aforesaid shifting from a normal monster to a boss monster by activating a Monster Shrine.

abilities

In Normal difficulty randomly generated bosses get one special bonus, in Nightmare two and in Hell three. Bosses from SuperUniques.txt have a fixed amount of special boni individually assigned in that file and will get additionally one random bonus per difficulty.
If a special bonus would lead to three ore more immunities the immunities granted by additional boni are skipped. If a special bonus grants more than one resistance they're applied in order Cold, Fire, Lightning.
Aura enchanted bosses use the very same aurae from Paladin's skill tree, but only one aura can be applied to one unit in a game. So if an aura is already assigned to a Superunique boss, a randomly assigned aura will be skipped. So the aura may change when you leave and re-enter an area with an aura-enchanted boss.

Bosses' Boni
Bonus	Effect	Effect to Minions	Bonus to Resistance	Notes
Bonus	Effect	Effect to Minions	Bonus to Resistance	Notes
No/NM/H: Normal / Nightmare / Hell
extra life	increases life by 300%/200%/100% in No/NM/H	increases life by 100%/75%/50% in No/NM/H	none	not shown at life bar
extra level	increases monster level by 3	none	none	not shown at life bar
extra fast	doubles (attack and) movement speed	dito	none	*
extra strong	increases AR and physical damage	dito, but less	none	**
manaburn	in fact a successful hit deletes all your Mana	dito	Magic +20%	for details see: Mana Damage
multishot	ranged attackers shoot three instead of one missile per attack	none	none	ranged attackers only since 1.10 emitted lightning from beetle monsters or lightning enchanted bosses aren't multiplied
teleport	will teleport away	none	none	if life below 33% (?)
cursed	chance to cast Amplify Damage on hit	none	none	see next table for `sLvl_AD` unlike to players also elemental attacks count here
aura enchanted	Paladin's aura, see next table	none	none	*******
cold enchanted	doubles AR, additional cold damage, Frost Nova on death	additional cold damage	Cold +75%	freeze bonus (chill) when 0% player resistance `4 + mLvl * 0,2` seconds according to Nefarius (1.11) `mLvl*5+100` frames
fire enchanted	doubles AR, additional fire damage, explosion on death ********	additional fire damage Nightmare and Hell only	Fire +75%	explosion 7,5% bis 10% of (SP) life
lightning enchanted	doubles AR, additional lightning damage, Charged Bolts in eight directions when forced into Hit Recovery	additional lightning damage Nightmare and Hell only	Lightning +75%	Charged Bolt damage `2 * [mlvl / 2] - 1` additional elemental enchantments causes the bolts to carry assigned death explosion damage
poison enchanted	doubles AR, additional poison damage, (invisible!) poison explosion on death	additional poison damage Nightmare and Hell only	Poison +75%	not shown at life bar poison length `mLvl*5+150` frames
spectral hit	randomly additional fire, cold, lightning, magic or poison damage to each attack	Nightmare and Hell only	Cold, Fire, Lightning +20%	see onderduiker's and brianc84's statement
magic resistant	increases Fire, Cold, Lightning by 40%	unknown	dito	not Magic and Poison!
stone skin	doubles base defense *****	physical +50%	physical +50%	.

* extra fast uses walking velocity from MonStats.txt and calculates percentage bonus like this:
max(min(2048/MonStats_walk-128,10),100)
thus it'll take the smaller value from either 2048 / MonStats_walk - 128 or 10 (min function) and compares it to 100; it takes the larger value then (max function). An unit with walking velocity of 7 will gain
max(min(2048/7-128,10),100) = 100
a bonus of 100% to walking speed.
Attack speed seems to be ignored in 1.1x, it might have gained a bonus in earlier patches, too.

** Atair and tommi list different values. This may be caused by negating the modifications of MonLvl.txt and other files. 'extra strong' Bosses gain a damage bonus of 150% via MonUMod.txt, which is modified by MonLvl.txt. Next there's a global damage bonus from DifficultyLevels.txt of 90% / 75% / 66% in Normal / Nightmare / Hell difficulties, respectively. Same happens to Attack Rating, omitting DifficultyLevels.txt (no global AR modifier). Also same happens to damage bonus granted to Champions (which also don't gain global AR boni). Have a table for survey:

Damage and Attack Rating for Bosses
difficulty	Boss		Minion		Champion
difficulty	damage	AR	damage	AR	damage	AR
'Champion' here means all subclasses, individual boni will step in additively
Normal	+135%	+90%	+67%	+45%	+90%	+67%
Nightmare	+112%	+75%	+56%	+37%	+75%	+56%
Hell	+99%	+66%	+49%	+33%	+66%	+49%

*** Calculating skill level for aurae of bosses was different in earlier patches. Special thanks to Dosbis for fiddling out correct behaviour for Patch 1.1x. Elder information is based on Jarulf and may have been applied up to 1.09 or even 1.10beta. Still, an aura level equal to monster level looks a bit ... odd.

Curse and Aura Level of Bosses
Skill	sLvl in Patch
Skill	1.09, 1.10beta	1.10, 1.11, 1.11b
sLvl: skill level `mLvl`: monster level `[ ]`: Floor Function
Amplify Damage	[mLvl / 4]	[mLvl / 5] + 1
Holy Fire	[mLvl / 2]	[mLvl / 6]
Holy Frost	[mLvl / 2]	[mLvl / 7]
Holy Shock	[mLvl / 4]	[mLvl / 8]
Holy Shock	only spawnable if `mLvl ≥ 20`
Might	[mLvl / 4]	[mLvl / 6]
Blessed Aim	mLvl	[mLvl / 5]
Fanaticism	[mLvl / 4]	[mLvl / 8]
Conviction	[mLvl / 4]	[mLvl / 8]

1.11 Uber Mephisto's Conviction is not based on his monster level. It is realm-only assigned, but player tests indicate his aura lowers 125% resistance, this equals sLvl = 20. Player's defense therefore decreases by 90%.

**** fire explosion follows these rules

damage is each 50% fire and physical
explosion radius is approximately four yards
explosion may be blocked via shield or claws; it is subject to Avoid and Evade (but not Dodge)
monster's Chance to do a critical hit is applied
damage doesn't increase with player count
level difference between exploding boss and its enemy is not evaluated
damage is based on boss' maximum hit points; the level bonus will result in a higher value here, but the life bonus is not applied
the global Corpse Explosion modifier from DifficultyLevels.txt of 50% / 35% / 20% in Normal / Nightmare / Hell respectively is applied

***** base defense is firstly doubled then percentage defense boni are applied if available; Amazon's Inner Sight skill will step in after base defense is doubled.
Tripling overall defense may have been valid in an older patch, or in Classic.

Bugs

There exist some strange constellations, which are applied to beetle monsters (Scarabs) as well as to lightning enchanted bosses, since that bolt-emitting stuff is the same thing for both.

Combination fire and cold enchanted: Frostnova and fire-explosion both are triggered two times each on death
Combination fire and lightning enchanted: if boss is forced into Hit Recovery each time the death-fire-explosion is triggered also, but it isn't graphically displayed
Combination cold and lightning enchanted: if boss is forced into Hit Recovery each time the Frost Nova is triggered also, but it isn't graphically displayed
Combination fire, cold and lightning enchanted: if boss is forced into Hit Recovery each time the Frost Nova and the fire-explosion is triggered also, but it isn't graphically displayed; Frostnova and fire-explosion both are triggered two times each on death
fire-explosion damage first was nerfed in Hell on closed battle.net, then with a client side patch (1.11), but not in Nightmare.
if a Flayer Shaman Boss is killed, he will spawn his carrier Flayer - since 1.1x also as boss and with same name and attributes like the late Shaman
in 1.09 multiple shot lightning enchanted bosses emitted more bolts, which also carried damage of any other elemental enchantment. Since 1.10 multiple shot doesn't work in combination with that bolts anymore.

nice to read

Monster Resistances and Immunities Guide, by onderduiker at the Basin
I know FE is nasty, but ...
Damage to Mana misconceptions, also Mana Burn, by Nefarius at the Keep

Masterdrop

Act Bosses drop from a special treasure class when the player defies them for the first time to enter next act. Because of a wrong quest flag in MonStats.txt Andariel and Duriel always drop that 'Masterdrop' if the character talks to any NPC in town after defeating that bosses.

miscallenous

I've skipped Champions and their subspecies completely so far.
Champions are recognizeable due to the blue-written term 'Champion' (or name of subspecies, see below), they don't have an individual name of their own. Like bosses they're immune to AI-bending effects. You may meet up to five subspecies.

Champions are generally better than normal monsters, simple Champ is most common subspecies
Fanatic Champions are more aggressive, less tough due to having their defense set to -70 (which actually means it is zero, and the attacker gains a straight bonus of 70 to his Attack Rating)
Possessed Champions don't suffer any curse and have a life bonus of +100%
Berserker Champions gain same damage bonus like 'extra strong' Bosses, they suffer a penalty of -75% to life
Ghostly Champions get their physical resistance set to 80%; if the monster class is already physical immune that bonus isn't applied, but if immunity is broken during the game, physical resistance is reset to 80%, regardless of actual breaking numbers; recent tests induce that resistance is regardless of displayed immunity indeed always set to 80%. Those Champs also get a chance to do cold damage (chill duration is 150 frames fix)

Hit Point values

One kind of monster will have its hitpoint value modified dependent of its hierarchical ranking according to the following table. All values are percentages.

monster	difficulty
ranking	Normal	Nightmare	Hell
Normal	100	100	100
Minion	200	175	150
Unique Boss	400	300	200
Champion	300	250	200
Berserker	75	62	50
Fanatic	300	250	200
Ghostly	300	250	200
Possessed	600	500	400

Credits

Anheixinren
Dosbis
brianc84
Nefarius
onderduiker
Ruvanal
Atair
tommi

Contact me via E-Mail, if you like.

D2library

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