Decima Angelus series by Bernard Bittler
Every fractal equation carries around an unlimited amount of worlds to explore. Even in generative graphics 3D fractals are often considered to be entirely different species. But Bernard Bittler found a way to fuse them into more conventional art. And it's a dark art with a twist.
In purely technical terms the result is achieved by using Mandelbulb 3D to create the fractal bits, MyPaint for actual painting, and GIMP for touch-ups and adjustments.
You can find more artwork by Bernard in his blog.