Ok, I did separate the darkness layer into the wall and the floor one, that way I can adjust both levels of darkness individually and get a bit more contrast between the two.
I did lower the opacity of the "darkness" layer on top of the room, as you suggested, and blurred the mask a fair bit more...

Here it is:
Click image for larger version. 

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Anything else ypu can recommend?



By the way jtougas, because you are interested in encounter design... I am now thinking about making the necromancer a passing Tiefling, that way he could stay back into the embers (Tieflings have resistance against fire damage) and let the fire keep the heroes at bay while he slings his spells...
I was also thinking about using a Tiefling bad guy in the next adventure, so I am not quite sure yet but it seems almost too good to let pass...