In this chapter, a more detailed explanation of the monster's AI, artificial intelligence, is given. That is, it will explain how the monster decide what to do. It will also explain in more detail how monsters act in general. Basically one can say that a monster can do 3 different things; move in some way, attack in some way or stand still in some way.

Just like a player, a monster will always have a specific action which it will be doing. Also, just as players, it must finish the current action before it can chose to do a new action. Some outside events may sometimes force a monster into a new specific action. Such events be the monster being hit and set into hit recover, a Fallen One retreating due to having a companion killed.

As explained in chapter 6, monsters, players and other things in the game are updated in a specific order and once every 0.05 seconds. During the update of each monster it will typically continue with the monster's current action and if it during the update reaches the end of that action it terminates it.

When a monster terminates an action, it will be set into stand mode. Stand mode is basically the monster doing nothing (note delaying is basically standing but for a predetermined time). The first thing that happens when a monster is set to stand mode is that it will check for a target. Thus, a monster always has a target, even if it is not activated and no enemy is in sight. For a description of target selection, see chapter 5.5.6. When a monster is in stand mode it will check for a new action by following a special script specific for the attack type the monster has. Note that a unique monster, and its mob if any, may have its attack type changed, see chapter 5.4, it will then follow the script of the new attack type. For a list of attack type for each monster, see chapter 5.3.3. The script may either end up in the monster being set into a new action type, or remaining in stand mode.

At any time, the game, for each monster, thus has complete knowledge of what target the monster has. It also knows where the target is and will as appropriate, calculate the distance to the target. It also always know what action the monster performed previous to being set to stand mode. In addition, while in stand mode, a special counter will keep track of how long the monster have been in stand mode. All this may often be important when going through the various scripts to decide the next action.

Within the scripts, some common tasks are very often performed either at the start or at the end. One of them is calculate the distance to its target. The game will also check the direction in which the target is located. This is usually used to determine in what direction the monster will walk. This chapter will not deal with path algorithms used, or explain how a monster walk. Many monsters will walk towards the target in the closest possible way, others will (at times depending on the distance to the target) instead walk around the target and so on. This chapter will generally just tell that the monster will chose to walk as its next action. Usually at the end of the script, if the monster is still in stand mode, it will turn around towards it target if necessary.

All monsters on a new level starts in a non active state (monsters with the Hidden attack type are an exception as they are always active, see chapter 5.5.9 under the Hidden subsection). As soon as a monster is within a players light radius and the view between the monster and the player is not obstructed by walls, the monster will be activated. While not activated, most monsters will not act although some exceptions exists, see chapter 5.5.7. Examples of such exceptions are archers and Succubi still being able to fire at golems or if a monster is attacked in any way. When a monster leaves the light radius of a player, it will continue to act for 12.75 seconds until it is set into non active status again unless it during that time, again enter the light radius of a player or is "activated" again. Some monsters with the capability of following players outside their current view will try to walk around possible obstacles to get to the former target when it get out of the light radius. See chapter 5.3.3 for more information about which monsters have this capability.

There are several different ways monsters walk in the game. The exact nature of how monster walking is done will not always be described in detail. Some general comments is needed though. Below are given very general description of some common walk behavior. They are given as a general information. Changes and diversions may exists. Also, if while walking the path is blocked, monsters may behave slightly different. Some times it will try alternative paths and other times abort its walk and reconsider its action. Some monsters have very specific walk behavior and those will be described within the actual AI script.

If a monster has the possibility to charge and it has been decided that it should charge, the game will check if there is really a path along which to charge that is not blocked by for example lava or a fence. If that is the case, the script will generally continue finding another action to perform as if it was decided to not charge.

When the game has decided to do a ranged attack, it will check if line of sight is blocked. If that is the case, the script will generally continue finding another action to perform as if it was decided to not do a ranged attack. Note however, that this check many times will not detect all hindrance which may result in the ranged attack being performed but yet hit an obstacle before reaching the target.

When a monster is to choose which player character to attack (if there is more than one present in the game) it will generally chose the one that is closest. However, the process of finding out who is the closest is somewhat non intuitive as the game does not calculate the true distance, but separates distances for the 2 main axis of the dungeon. This is most easily explained with an example.

1. The dungeon is divided into several "squares" of which each square can hold a monster, player, or some dungeon inventory. Call the axis of squares running from the upper left to the lower right X, and the one running from lower left to the upper right Y.
2. The game will then calculate the distance to each character separately for X and Y direction.
3. It will then for each character use the largest distance of X and Y.
4. Comparing the distance got from step 3, it will attack the one with the shortest distance.
5. If two characters are equally close, the one that has first entered the game will be the target. It is thus wise to have Warriors enter and create a game, then have the Rogues enter it and finally the Sorcerers. If someone leaves the game, the next person to enter the game will take the place of the character that just left, not the last place.

A monster will, of course, only consider those characters, including golems, that are within its current visible range and are not out of sight due to a wall or other obstacle. Golems are only targeted if adjacent to the monster or if the monster is a skeleton archer, goat archer or succubi at any distance.

If the target ever disappears form the sight of the monster, it tends to walk up to the last position the target was seen. If the target is still not visible, it will try to pick a new target. Some monsters, as noted in chapter 5.3.3, have the ability to follow a target even if it is out of sight. In that case, they will not walk up to the last position of the target or stop, but will instead find the closest way to the target if it is not too far away.

Diablo and Hellfire uses a pretty good algorithm for random numbers, unfortunately it is at times used improperly which result in somewhat non random results at time. One of those cases exists in the AI scripts of Spitters, Magma Demons, lightning Demons, Bone Demons and Diablo. Contrary to other AI scripts they use Rnd[10000] in some cases. Unfortunately the algorithm for random numbers only use 15 bits to calculate random numbers (it uses 32 bits internally but the random number is based on 15 bits only). This has the side effect that large values used in Rnd[x] will not be evenly distributed. In the case of Rnd[10000] lower values will be more common as there are really only 32 768 possible random integers. This is further complicated by the fact that the game then as a second random number in those AI scripts mentioned above uses a Rnd[100] but still compare the result as if it was a Rnd[10000] meaning the result will always default as if it was a low random number.

A further problem is the fact that the game reseeds the random number generator before processing the monsters. This is done to keep the game better in synch in multi player. Unfortunately this also cause problems in that monsters will not always act as they are supposed to, cause long streaks of the same result. An example is a monster missing continuously for almost an infinite time when they in fact should hit way more often due to having a auto To Hit for example (see chapter 5.1 under To Hit). This only affect monster behavior though, nothing else like item drops or the behavior of spells.

In an attempt to keep monsters more in synch between computers in a multi player game, Blizzard has unfortunately used a rather strange and incorrect way of reseeding the random number generator in Diablo right before processing monsters (technically each monster has its own seed used for its behavior and it is the seeds of the monsters that are reseeded). This may cause the familiar result of a monster getting "stuck" with a particular behavior or result both when acting and when attacking. This is unfortunate.

Here each of the different attack types will be described. Many monsters, but not all, have special behavior types of an overreaching nature. Often composed of several actions performed in succession either until completed or aborted for some reason. They will be explained separately at the start of each script. A general section describes things usually performed every time. This includes a monster under an overreaching behavior.

The step by step instructions to follow are divided into several sections depending on the distance to the target. Use the appropriate one. Some times further conditions may also be given. For the specific step by step, you should use them in the following way. Always start at number 1. If at anytime it says goto #, where # is a number, immediately move to that line and continue from there. If it at any time says exit, the script has finished and you should not read any more. If you exit or reach the end without a new action being set, the monster will continue to stand and will check again 0.05 seconds later, that is during the next update, see chapter 6.1for more information about how the game updates monsters and other things.

The scripts will only refer to attack options such as melee attack or ranged attack. For information about what type of ranged attack monsters have, see chapter 4.2.

Finally note that although quite detailed, there might still be many special cases, exceptions and other smaller facts omitted. It is still my belief that it is a comprehensive explanation of each attack type that will cover almost all cases.