Why do monsters attack certain targets?

[Crystalion]

Everything in the game has a threat rating, which determines the order they're attacked in by NPCs/monsters. ..

I'm not sure if there are attacks that will change your threat rating, but it seems likely, considering how monsters will go after Fire Golems and Holy Freeze mercs after even if there's a higher threat rating something around.

Also, once a monster/npc is locked onto a target, it'll stay on that target until it attacks or something interrupts it... then it'll go to the closest target with the highest threat rating.

Well, this is fascinating subject and you've made an excellent start, however, even without code readers enlightening us it is still possible to empirically convince myself that some of what you say here does not sufficiently model actual monster behavior...

Convert (& Mind Blast), Confuse, Attract, minions with threat factors (e.g. Decoy) *and* proximity are all very interesting to see interact.

Here are some suggestions (not definite "facts") based upon my observation of the above as to where some "holes" might be in your model...

Monsters don't really "lock onto a target" per se, afaik, but instead "commit to a current action". This distinction is important for prediction. If I click on a monster (to attack it with a melee type skill) the game will helpfully path me toward the target and once I'm "in range" will execute my attack. If I hold the mouse button down the attacks will helpfully repeat. This feels like "locking onto a target" but isn't really--and is more so than what a monster does!

That "pathing" part can fail (in large number of ways). If it fails, or after the attack is executed, the monster AI again "decides" what to do (another "current action"... monsters also have an idle state, so their action commonly can be "huh? I'm supposed to do something?" but this AI choice seems rare when close to potential targets for *most* monster AI).

So we can think the monster is then "locked onto a target", but afaik this is really an illusion fostered by the likelihood that the monster is now very proximate to the target, and will thus decide to re-target it.

You can see the power of proximity in AI choice by simply moving in and out of the space between a monster and its "locked on" target (if you don't have multiple copies running or a friend to help you experiment you can still position your minions or make use of the act 5 barb horde in Bloody Foothills).

In my experience, despite the threat level factor, a Decoy won't pull a monster off of you without proximity working in your favor. However "Attract" can (there is a limited radius for this effect, but it is amazing to see monsters just pull off their targets and walk yards away to the attractive target). Attract also lets you see, since the victim's AI is then biased, that being hit can cause a change in (adjacent) target selection...

When something like Holy Fire touches a monster (high range Static Field, etc. etc.) it may very well be "waking them up" from the "idle" state. In this case it appears that you automatically are given top billing in their hate list for deciding their post-idle current action (i.e. to attack). One of the reason AoE knockback effects are so studly is that a knocked back monster is reset, and must choose what to do again. I believe this means a bowazon using knockback is magnifying the power of her Valk/Decoy/Hireling/mates to tank (because the monster will not, in fact, beeline for the zon as a non-knockbacked target would). (I should note that 100% monster flee, combined with slow/chill/knockback is pretty effective... akin to EQ's snare and reverse kiting tricks).

Watching this kind of behavior leads me, ala Occam, to believe there isn't a hate "list" in D2 at all (perhaps one psuedo entry slot for last person to damage me?... poison and open wounds though, which *do* remember their source, don't really seem to keep such an entry live/current).

Depending on whether this implementation exists at all, it is interesting to wonder if the "slot" can be overwritten (e.g. perhaps by a higher net damage attack?)

D1, fyi, had a bias here, iirc, based on game character slot (0..3, based on entry into the game order, or reuse), which some players learned to make use of for tanks.

Converted monsters are interesting as well (no time to delve into that now... did you know that even though you can't target them they will still die from having been previously poisoned by now "friendly" sources).

EQ-like games have a powerful "feign death" method of blowing off monster's attention (aka "lock" aka "hate")... D2 has that also: the town portal spell (has side effects, of course). (Also stun/knockback/pathing blockage etc. etc. but I thought I'd put in another plug for TP, "the most powerful spell in the game"™)