Destroy monster actions

How do I make it so that if I destroy a monster an action happens like a wall opens or another one spawns?

Depends on the port (if any).

ZDoom's hexen format supports thing specials, so you can assign one of these to a monster that is activated when it dies.
Alternatively, you can use dehacked to assign one of the many death specials to a general type of monster, or you can simply use one of the default death specials like the Arachnotrons and Mancubi on map07, and the Cyberdemon and Commander Keens on map32.

IIRC: The Cyberdemon doesn't do anything special in map32 and Commander Keen's death can trigger events on all maps you place him in. Just tag the sector you want to be raised with "666".

Using (W)hacked you can give the special 'keen ability' to any monster.

I'm not sure what exactly it is you want to do.
If you just want a monster to spawn and replace a fallen monster, the easiest way to do so is to let him teleport in to the same spot the original monster is standing. Make sure the line is "teleport monsters only", this way he can only teleport once the first monster is dead and no longer blocking the teleport destination.

t.v. said:

IIRC: The Cyberdemon doesn't do anything special in map32 and Commander Keen's death can trigger events on all maps you place him in. Just tag the sector you want to be raised with "666".

Oops, you're correct on those two. I was under the impression killing the Cyberdemon opened the inner exit door, but it seems it's just a normal door. And yeah, the keens can be used on any map. I should especially know that, since one of my maps uses them. :P

I've discovered in some of the scripts I've done that including code that damages the player's health don't seem to work. Now, I know it can be done, but I don't know what I should be doing different. I have the player tagged to the script and it will work on any other monsters I tag to it.

(I thought this was a good place to bring this up.)

Hyabusa said:

How do I make it so that if I destroy a monster an action happens like a wall opens or another one spawns?

In vanilla Doom you can spawn a secondary monster by replacing a missile with said monster and shooting it with a specific codepointer (eg replace the Arach plasma with an unarmored Hitler to spawn on top of a dead mecha). Make sure it's NOT bitted as a projectile, that it's of like width and that the spawner is deemed passable! Doom risks crashing or locking up if these stuff are not taken care of. In ZDoom or Eternity you can just use the A_SpawnItem or Spawn codepointer.

Hexen from the very start has the ability to let things triggering stuff by death, so does ZDoom. You can use one of those thing spawning linetypes. In Eternity it's not implemented. You probably have to add a codepointer which calls a small script which then executes what you want. You'd need a small script compiler then...

for the first question of this thread, that's easy. To trigger an action when a specific monster dies, you need to edit the properties of the thing (the monster), put 80 in the "action" and your script number in the script box. Then just write the script you want for a special action (like opening a door or something).