Eclipse looks very similar to 's demon design, like for example the claws, 2 toes feet, dark colours, no nose, dark eyes with glowing pupils, spiky hair, long tail with an arrow at the end, the demonic red strips all around his body and some demons have horns.
I suggest you find the person who drew Eclipse and notify shadyever about Eclipse's design.
I'm pretty sure you're over reacting about this. My opinion is that Eclipse is more based off of Mephiles' design with more of a demonic look. That tail isn't an arrow either, it's more like the barb on a fish hook that prevents a fish from being able to escape. This is a creative design that wasn't copied from anything. Try to analyze more before pointing fingers about a "similar" design.