Thank you! 
(I hear you mate, this Conan issue arrived in Italy in the early '90s, when I was about 16yo, and it was late in its run: I much preferred its previous years, but in the covers, at least, Buscema still inked himself, and he was great!)
The scene and setup are really simple, if a bit tedious: I sculpted the scene in low / mid density in 3DCoat, and converted it to, like,10M triangles in Blender.
That res and definition makes it really not finished, as a sculpt, and if the sculpt were the object, I would aim for at least 40M total for this piece.
(For instance, this sculpt fails badly to support the shading of the face).
But for the purposes, I decimated to 1/10 of the triangles to ~1M of them, and add a smooth modifier on top on that! (counterintuitive, but needed, to have smoother shadows from the shadeing, and to be able to actually ink in GP, on my low spec PC).
This is the result:
Then I did the cel shading:
The scene is separated in meshes by color, so I don’t have to do any texturing, but only carefully place lights on the scene to make the shader’s ramped colors to “appear” as I needed.
Here’s a material, the horse’s body:
Then I add the manual grease pencil inking and the gp lineart modifier (that is not active in this screenshot, as it’s camera dependent):
In the inking you see I added white and blue strokes where needed on the hair, because the shading was not sufficient (the shadows were too prominent).
And besides, those hairy areas on the hooves, while they work in the voxel sculpt, don’t have enough geometry to live here, these are just bad, my mistake, I’ll possibly go back on them.
Plus, the vultures in the sky I did as a separate grease pencil.
And the wind or vapour dynamic effect I did by mixing a shader like the previous one with a transparent one, driven by a color attribute.
I exported in ReC2020 / AgX to support the final color grading outside Blender to my taste, but before that I tried to be faithful to the colors of the ref (more or less: I had too many moving parts related to the lighting to be actually very accurate):
Finally, I noticed a couple of differences on the pose of the sculpt, in relation to the ref, but as they are not errors per se, in the sculpt, I left them.
Is grease pencil just used instead of texture painting, and what’s the benefit of that?
So yes, and the benefit is that I don’t need to do UVs, plus, texturing, on my PC setup is extremely slow, while GP, in surface mode, on a ~1M triangles scene, has just minimal lag.
The cons of GP, imho, is that you can’t use shaders on them, and its UI/UX could be less convoluted.