Grimrock

I want to change them to make something else. I can't find out how they work with the asset pack though.

Do you had a look in the original map:

viewtopic.php?f=22&t=8404&hilit=original+map

I did, and can not find a script that is responsible for their action. You can place a simple beacon furnace in a new map and spawn 4 power gems near it and it will create an essence of fire without doing anything else.

The makes it work. My guess is you'd have to define a new altar object without the BeaconFurnaceController but add a generic ControllerComponent. Keep all the other classes in the definition. From a script, you could activate the particle systems using the controller when 4 gems (or ?) are inserted on the altar and then spawn what you want it to create.

Edit: also, use playsound() to play the 'beacon_furnace_activate' sound.

If you define an object as "essence_[string]" and enter "[string]" in the essence field of the controller in the editor, it will make that object. So you can, say, define an "essence_cow" item and type "cow" for the element. You could even give an onInit hook to essence_cow that spawns another arbitrary object and destroys the essence_cow, if you really want to make something that doesn't start with "essence_".
I don't think you can isolate the gems/particle animation from BeaconFurnaceController, and recreating the fancy animation by hand without having the original code sounds very difficult.