Treason of Tzeentch

Pick 1 enemy unit within 18" of the caster that is
visible to them.
Roll a number of dice equal to the number
of models in that unit.
For each 6, that unit suffers 1 mortal wound.
If any models from that unit are slain by this
spell, subtract 1 from hit rolls for attacks
made by that unit until your next hero phase.