Basically the idea with this build would be to find a group of monsters (preferably a big one) and throw out a few Haunts, Mass Confusion, and then run into the group and pop Spirit Walk. Immediately after you Spirit Walk, cast Soul Harvest. Soul Harvest deals 275% weapon damage as AoE around you and will also cause the Spirit Walk to end, procing Umbral Shock for an additional 236% AoE. All that damage is also increased by Mass Confusion (and, after the first time, by the Soul Harvest buff) as well as the Pierce the Veil passive. If anything is still left alive, you should be able to finish them off rather quickly or, if there's still lots of monsters around, you can pop Sacrifice to take them down (and increase your damage even more for the next group). Thoughts?
It seems like your build makes the player do a lot of work for very little 'value added'. What I mean is, why cast this fancy combination of skills (mass confusion - > spirit walk -> soul harvest) when you can just spam firebomb with the red rune? Every build needs a damage component. In your case your damage comes from runed support skills. I may be wrong but I don't think you can make successful builds whose damage component comes from the runes on support skills. These runes seem like nice bonuses but I don't think they can form the main damage component of a build by themselves.
Well it's a gimmicky build obviously. It's definitely never going to be the most powerful build you'll see around here. I just thought the Umbral Shock/Soul Harvest combo (511% WD AoE) was kind of interesting. The CD on Spirit Walk and Soul Harvest are 13 sec. with the Spirit Vessel passive and you also have Dogs and Haunt to fill the gaps between CDs with the build as I suggested it. The other 4 skills could really be used for anything: single target damage, Gargantuan, better CC, whatever.