Pets and traps are fun, but builds relying on them don't seem to see as much action as one would expect with the potential they have. One reason for this may be that people just don't know a lot about them... and some things are still not known for sure.
Therefore I would like to create here, with your help,
A definitive collection of proven knowledge about how pets and traps work.
- especially how their damage is calculated. This includes what bonuses affect them and how those, in turn, are affected by things like difficulty balance modifiers.
For general info, there are other threads already in existance:
A "complete pet guide" with similar intention had been started here, for example, though it turned more into a discussion about builds. There have been numerous threads featuring pets in general, as well as specific guides like Tyr's work about the Nature mastery, to which I once added some thoughts on how to use different pet damage types effectively.
The life scaling and resistance bonuses throughout the difficulties has been stated by Apocalypse80 [and confirmed].
When it comes to damage, however, the opinions often [used to] differ, as do the tests.
So this is to be [and has been] clarified. Any additions are welcome.
- The only actors affected by pet gear and other pet-specific bonuses (Master Mind aura) are actual pets summoned by player skills, permanent and temporary. (Herafter referred to as "pets").
Artifact summons, scroll summons, converted monsters and also traps are instead regarded as allies (like other players), so they only receive general group buffs. Briars don't get anything, since those are mere "barriers". The Monster Lure is classified as a pet.
- Flat damage bonuses from pet gear affect melee attacks only. This includes the Wisp's zap. Lich and Nightmare throw ranged spells instead, so they don't get to use them - except for rare cases when they decide to punch an enemy already in melee range. The Nymph's arrows were changed in the fanpatch in order to carry all bonuses.
Trap bolts are also projectiles which carry flat damage bonuses, and while not affected by pet gear in the standard game, they do benefit from general buffs (e.g. Battle Standard).
- Relative % bonuses from pet gear and auras, as always, work on ALL fitting damage types caused by the boosted creature in any way.
- Likewise, pet intelligence does increase all associated damage types (vitality, lightning, fire, ice and burns), including those granted by gear. Str would increase their weapon damage, but since no standard pet uses a real weapon they at least do not benefit from it.
- Pets and Traps receive difficulty level damage bonuses to all damage types (+75%/+150% in epic/leg, except pierce damage and DOT which get +60%/+110%). Those are added to any other % increases (whose relative benefit therefore becomes less with increasing difficulty).
Ironically, traps get lower difficulty bonuses to piercing damage than pets (+30%/+60%, barely compensating monster resistance increases), but higher physical ones (+100%/+200%) - because they scale like monsters.
- Both pets and traps receive difficulty increases to resistances, health and AO/DA (see Poinas' answer below). An interesting result of this is that the boost of up to +400 base health makes even level 1 pets rather sturdy later on, while the free +500% dwarf other % health increases (like Heart of Oak) in comparison.
- Melee pets are subject to the usuual OA/DA effects and can inflict critical hits, which further increase all damage.
- A bug: Pets don't use pet skills above maximum level. Not even passive ones! This has been worked around in the fanpatch by raising the max level to the ultimate level, which means you can now spend real points up to the former max+4 value, but not apply any +skills to go higher than that.
- The flat +pierce damage from the Blade Honing aura does not work on pets or traps like it does on allies, probably because the entire buff requires a weapon.
If you wear this "physical" petmaster ring...
- 70 physical damage is added to all melee attacks, and Nymph arrows since v1.17.
Those get increased by any +%physical the pets may receive (including difficulty bonuses).
- All physical damage inflicted by pets is raised by additional 12%.
This also affects the 70 that were just added.
- All pets get 40 armor, so they receive 40*0.66=27 less physical damage per hit.
This becomes less useful vs epic and legendary monsters.
- All pets get 100 extra base health.
This becomes much more later on due to the large difficulty multipliers for health.
If a Champion is wearing three pieces of "Ritualist" jewelry (+270, +36%)
with Wolves that have max+4 Survival Instinct (+110%)
and max+4 strength of the pack (+90%)
and there is an active Battle Standard nearby (+274)
while playing in legendary (+150%)
then the wolves will inflict
(100+36+110+90+150)% = 486% of their natural damage
(210+274)*4.86 = 2352 additional damage on every hit.
This value could go above 2700 with a fourth Ritualist item (a staff) - It's just that with all the physical bonuses around, Champions usually prefer a melee weapon.
- How exactly are the bonuses included in critical hits?
This should work like it does on every other creature, meaning that the total damage output recieves an additional multiplier depending on how much higher the OA is over the enemy DA.
The only thing that would need to be cinfirmed is whether pets crit at all, especially when not carrying a melee weapon.
The following refers to the original post, at a time when all this info was being collected. Now that it is complete I edited this part to just display the results.
Thanks to all who contributed!