Really awesome piece, it quite captures the feeling of the original Heretic. As you said in comments below, the original game had some rather lousy lighting and was in my opinion too... animated? Bright graphics here and there and stained glass, so colorful and whatnot.
I like your darker take on it, it gives it a stronger Medieval/Gothic feel which I think is that Heretic was aiming for. The Ethereal Crossbow looks really neat as well, though it could also be confused with the Iron Doom spell from Heretic II. Regardless, it looks well, powerful and that Undead Warrior in the background is a cool, creepy touch.
I find it fun that you made the main character female, but she fits quite well with the scenery and the pose is very well done. Also that cape is awesome.
Well, honestly, that does not look very recognizable, except the gargoyle. But all that could be in H2 as well, I don't know... Now I can guess there the Ethereal Crossbow, also an Undead Warrior... But, certainly, good job!
You bring up a good point. The initial idea was to make it faithful, but that's when you run into problems. Heretic has an old, primitive lighting system, where places with no light can be still brightly lit ambiently, which wouldn't make sense when painting. Ice Grotto level is one of these, all the separate 'houses' within the castle walls are under a solid overhand, but are lit brightly enough to see. That's even before things like lava rivers within ice fields. Picturing it realistically it would have been far darker, so that's what I was kinda going for. Basically my own take on the original. Glad you like it!