Hey /u/Mark_GGG, just to clarify, will Archmage work with Reap/Exsanguinate as long as you add a mana cost via Voidbringers or Apep's Rage?
All skills have costs, they just are sometimes zero - not having a cost just means the value of that cost is zero. Since Reap's base mana cost of zero is (probably) lower than a % of your unreserved mana, Archmage will raise the mana cost to that value. You don't need any uniques.