Well I haven't seen the Doomstone used in a Daemon army as of yet. The reason I never used it it only works on characters which makes it a little pointless.

However taking an old play of using the Calladius with the psychic battle squad you can possibly get some use out of this artifact.

I see the unit that benefit the most is a hound unit with a khorne herald. The reason why I think this is a good tactic is the fact that hounds can engage multiple units rather easy and the character cant hide. What you do is before you assault a unit with a character you really want to kill you first lower the units leadership to two. Seeing that some models like Draigo may be hard to lower leadership.

Once engaged you are permanently lowering leadership if they fail their test. With the battle squad you can lower leadership to 2 which would easily kill off that hard to kill character. For example it can be that captain with storm shield with eternal warrior and a bunch of other crap. The beautiful part is you can kill the character before it can do anything. If it fails deny the witch and your battle squad power passes all you have to do is touch the squad and all the characters die.

This can be a big issue if for example you have an Inquisitor, Draigo, Tech Marine, and a bunch of other chracters in the squad. You can kill off all the characters leaving only the actual squad left to fight.

It is rather good vs ig as well if for example you face the 5 sargent squad with a commissor. With the psychic squad lowering leadership to two. The characters can poff be gone. Since it's not a morale check stubborn does not defend against it. Which means if you win combat that squad is leadership 2.

Even without the squad the Doomstone can be useful in forcing lowered leadership vs squads who have a lotof characters in them. On average you are lowering leadership by 3.