Create enemies which summon other enemies on death?

I recently started playing through Valient.wad and loved how arachnotrons could turn into acracnorbs when they die, does anyone know how they did this? i want touse it in my maps.

I haven't looked into Valiant deeper, but I'm guessing they did it using Dehacked by connecting the last DEATH frame of the arachnotron to the first SPAWN (or IDLE) frame of the arachnorb, with appropriate animation frames to make it into a smooth transition.

Here's a good introduction into doing Dehacked modifications by Doomkid:

If you use a format that supports DECORATE, you can do it somewhat easier. If you open Valiant in Slade3, you can look into the DECORATE lump and see how monsters can be defined. There you can also find the "ValiantArachnotron" actor, and see that in it's DEATH2 state there is the function "A_SpawnItem("ValiantArachnorb", 8)" which basically spawns an arachnorb at the right moment.

Here you can find tutorials on getting started with DECORATE:
https://zdoom.org/wiki/Tutorials

With dehacked, when the arachnotron dies it has a 50/50 chance of jumping to a different death state, which calls the spawn function to spawn an acrachnorb. I haven't looked at it in while but pretty sure that's what it is. 

I'm familiar with Slade and adding mods to wads, although it seems Valient.wad is organized Differently(better) than my current test.wad. all of the DECORATE is stored on one file together whereas I've just been copy and pasting all of the files from mods of realm667, and therefore have many DECORATE files. can I just drop in the "ValiantArachnotron" and "ValiantArachnorb" and change the names?

You can have as many DECORATE lumps as you like. It's cleaner to have only one, and a good practice to get into. I mean, they would all be named DECORATE anyway so you couldn't differentiate between them except by their location. So yes, you can just copy paste them and change the names, and any other stuff you'd like to alter. Keep track of the stuff you use so you can credit them appropriately when you release your work.

The monsters use the MBF-added actions RandomJump and Spawn. This is what allows to have a 50% chance and to spawn some arbitrary item instead of being restricted to drops.

Note that without MBF, you can still get monsters to spawn other monsters on death, but it'll happen all the time and it'll hijack one of the zombie drops. Mods like All Hell is Breaking Loose use this, to say nothing of The Sky May Be/Blessed Engine.

Valiant features DECORATE equivalents because, at the time it was made, Skulltag/Zandronum did not feature MBF-DeHackEd compatibility yet.

Never seen in Valiant that arachnotron spawned an arachnorb.

I feel lucky