It's called wet blending, it is quite difficult to achieve - it's done by putting a thin layer of paint on wet and then taking a 2nd brush and dampening it slightly and using it to 'blend' thin the paint out ward into the colour below, it's a useful technique for large smooth areas
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~The ravings of a single mad Goblin is bad enough, but such a power-hungry, malice-filled creature as Mortis can never hope to be understood~