I like that you have essentially created a weighted Cast Random.



For the Necrotic Strike.
You might go something like:

Mapped Key: Necrotic Strike
No Hotkey.
Four Steps, One per DK.
Each step, sends the Necrotic Strike key bind to one slot.
Prevent a step from advancing for a GCD (or whatever).

Then modify your existing DPS.
So the Necrotic mapped key has priority over other dps moves (ie, is listed first).

Something along those lines.