Undead only?

[Gatts]

For the ghosts I would like to add:
after combat it is handy to delete / disband more of your ghosts than advised (let's say your limit is 100 ghosts, you have 120 ghosts... delete/ disband 30 of them... so you would have only 90 -> this will give you one additional round of control in most of battle, is some you can control them even longer... as ghosts ability to suck soul is much less powerful as vampire ability to suck blood)

In addition I used strategy of not-attacking - meaning I have moved ghosts in front of enemy units to attract or strikes/shoot attacks (to protect my backup units / weaker units) -> where you may decide if you DEFEND the ghosts (press space/D) or leave them one action point free and wait them (W) -> I let them wait if the enemy is too strong (so my 90 ghosts get 8 killed by first enemy and suck back 12.. .but are hit again by four other enemies and are reduced to 50 only... now I can freely use them and exploit their stealing soul ability -> as now I really DESIRE that my ghosts will grow back to 90/100 units... weaker the enemy is, more ghosts I get... )

Also combine ghosts with magic spring and Iron skin.
If you have many of them - move them in near enemy with fire or magic attack.

But basically when I had only 90% of ghosts in my active stack - I was able to hit twice (one counterattack + one attack) before they went frenzy/crazy... then I put gloth's armor on them, so they would stay near enemy archers.

But do not try to use ghosts on impossible:
They either raise above 100% of your leadership and will start directly hitting your own units

Or they would die too fast (even as they have 50% phy resistance, on impossible are inquisitors/ priests / and many fire damage dealing units quite strong - where the HP of ghosts will be quite fragile -> low compared to their leadership)