A mix of glowing eyes and the traditional style, which I think looks suitably heretical.
Paint the eye with a solid coat of Khorne Red . Glaze this below the eye too, in a rough oval. You can wash this into the recesses too.
Mephiston Red in the first half of the eye, but also slightly as a line around the bottom (think Nike tick). Glaze some of this inside of the previous glaze under the eye. Might take a couple of light glazes to build the colour, don't rush.
Use Evil Sunz Scarlet , leaving some of the Mephiston, including a bit of the tick. Glaze under the eye again, closer as usual towards the eye.
At this point I use a bit of Nuln Oil Gloss with a dot of Abaddon Black to darken the far corner of the lens, over the Khorne layer. I also line in the top of the eye to define it.
Troll Slayer Orange into the the previous Evil Sunz Scarlet layer, again leaving some this visible. This is getting smaller and harder. Thin paint, use the tip of brush. You're painting not much more than a dot and a line at this stage that follows the curve of the eye.
Add a dot of Yriel Yellow into the orange. If you can, an even thinner line in the orange. This is incredibly fine.
Add a dot of White Scar in the previously darkened red.
Add a coat of Contrast Medium to even the finish over the lens.
Glaze some Flesh Tearers Red in Contrast Medium (probably at least 1:3) on the red eye glow from before. Start light and build up to get the intensity of red without it being an obvious line.