Well, I love this idea.

One concern, won't it put Empyreans even FURTHER above everyone else?

Blade: Hi: Darkness -> Ukko's Fury: Light = Void

Light is already completely soloable even without meditate/sekkanoki on WAR and generally doubles WS damage not to mention whatever lucky bastard gets a MB off of it. If this does any more damage you could two-hit Sobek!

On the other hand if it maybe required three people, as in say add a filler skillchain (void) to go something like this: Darkness + Light = Void(0 damage) then you can stack that with another skillchain: Darkness + Light = Void stage 2 = Cosmos!

Maybe give it something more than just damage (Since you really don't need to be doing more damage than most Empyrean WS anyway) like a Terror or simple Stun effect?

However the real problem is that no NMs really require you to skill chain, in that its faster to forgo all 'save tp' strategies and just use TP at 100%. To accompany this lvl 4 skill chain the real issue that should be implemented is a type of mob that takes severely reduced normal damage, but incredibly high skill chain / magic burst damage.