It has probably been mentioned before as I have seen several topics in the past on this subject, but I think it would be very wise to revisit Murderous Prowess/Mastery for Total War: Warhammer III. I find it really strange and unfitting that the Dark Elves' faction trait is also affected by their deaths, which contradicts the ''prowess'' and ''mastery'' in their essence. There is no prowess or mastery if the unit gets killed.
Parallels are often drawn between Khaine and Khorne, and if any semblance of Khorne will be introduced in Warhammer III, Murderous Prowess might take away from a theme that better befits the Khornate, rather than the Khainite. We both know that the main difference between Khaine and Khorne is that Khaine allows for a level of control for the Dark Elves, through murder and fear, whereas Khorne is more indiscriminate on the field of battle and does not care from where blood flows, as long as it does.
To keep this post short, I would like to place my suggestions to more accurately portray the Dark Elves' faction trait underneath:
1. The easiest way would be to make it so that Murderous Prowess/Mastery only prock based on a total amount of *enemy* casualties, and is unaffected by the amount of slain Dark Elves.
2. A slightly more complex way would be to remove the global resource bar, and make murderous prowess and murderous mastery unit specific, wherein a unit may activate the same buff but individually, based on the amount of *enemies* slain. Instead of a global bar/ability, the trait would be localised.
I understand that CA originally intended to make the Dark Elves and High Elves as contrasting as possible, even in the faction traits, with one being strong initially, and the other being strong later. You have done so quite admirably, but I fear you may have overlooked that one crucial element in your design of the Dark Elves. I believe that, with either of these suggestions, the nature of the Dark Elves' faction trait would remain unchanged, while being more truthful to the way Dark Elves worship Khaine on the field of battle both lore-wise and in-game.
I hope you will at least consider my suggestions and thank you for your time.