In Diablo 3, does the Valley of Death rune add a 15% modifier to the already 15% additional damage of Marked for Death, bringing the total additional damage up to +30%? I can't seem to find anything that talks about this. All the other questions that I see are related to multiple instances of Marked for Death stacking. I just want to know how that specific rune stacks. Does it? Or does it just provide +15% additional damage to everyone within the designated area?
Valley of Death transforms Marked for Death from a single target to an area-of-effect on the ground. When you target a mob with the skill with the Valley of Death rune, it's just dropping the AoE at their location. This is no different than using the skill on an empty area. This means that there is no target for Marked For Death to receive the initial +15% damage modifier and any mobs under your cursor only get the debuff from the AoE on the ground.