Coalescing Shadows interacting passively with the same spells as Surge of Darkness is a bit bland. The problem is compounded by the fact that you don’t really have agency over the orbs at all. You will never hold a Mind Blast until you have 3 orbs and Mind Spike will consume all orbs passively.
I would like to advocate for coalescing shadows to interact with Devouring Plague and Shadow Word: Death.
An example implementation might be:
When you have 3 coalescing shadows, Shadow Word: Death consumes all shadows and deals damage as though the target is within execute range.
devouring Plague consumes shadows to deal increased damage.
Consuming shadows reduces the cooldown of Shadow Word: Death by 1 second per shadow consumed.
This would add additional layers to the synergy between mindbender and shadow word: death through puppet master and shadowflame prism.
Ideally, shadow orbs should be generated only through mind blast. Harnessed shadows would allow mind flay and shadow word:pain to generate them as well.
Shadowy insight could be moved to the left side of the tree to juxtapose it with mind melt. We could also have a followup talent for shadowy insight which allows devouring plague to have a chance to proc shadowy insight based on coalescing shadows consumed. This would add another nice positive feedback loop of insight generating orbs and devouring plague consuming them to generate more insight procs.