This effect was generated in Filter Forge, a Photoshop plugin that allows you to create your own filters.
This is the fourth in the Monster Spawn series. This one was again, a routine taken from 'The Monster', but this one was further enhanced by additions.
This filter alters color tones and relief or height.
the color tones are altered through the use of a hue/sat component and an impulse curve component.
The relief or height mapping is altered through desaturate, levels and gamma components.
Craig Kraellin Walters monster spawn relief height color tone mapping surface