What does the revenge ability do?

[Master_Penguin]

I've been looking at many different sites online but none seem to answer my question. Abilities... What do these do?:

1-Revenge.
(you can find it via - Units > Passive > campaign > space fel orc)

2-Spy.
(sorry, haven't found it)

3-Dark Conversation and Soul Preservation.
(Undead > Campaign > Heroes > Mal' Ganis)

4-Cargo Hold Death.
(believe the Goblin Zeps.)

There is probably more.

 

[rysnt11]

3 to make any villager stored in a some place, then use sould preservation to turn them into zombies

 

[Tleno]

Revenge was originally used in that secret tower defense level, I guess it just attaches the missile model to a unit or makes it explode on death (graphically, without dealing damage).

Spy? I have no idea.

Dark conversion is from Reign of Chaos human campaing, these are used to transform villagers into mindless zombies.

Cargo Hold Death makes all the passsengers of vehicle die when it gets destroyed.