Custom ability - how? (Art and functions)

[Aphystic]

So I'm working on an ORPG with a friend of mine and we're both rather new to all this and we're at the stage where we wanna create our own heroes, right. I wanna create a holy shotgun (very like the one in diablo 3). It should be a passive or toggle and every autoattack should release 3 waves of holy light in an arc (doing aoe damage if possible, otherwise single target). So am I able to make the art myself and import in the world editor or am I screwed? And how am I supposed to make this spell work like I want it to?