Alistair's Arcane Absorption

You reach out with necromantic power and sap the arcane energy from any nearby casters; (up to 3 targets) any targets must make a CON save or take 2d6 Necrotic damage and for the next 6 Rounds; or until you drop concentration, any targets have disadvantage on all spell rolls. On a successful save the target(s) only take 2d6 damage.