hmmm that one is nice too.. but berserk would make sure one army would be gone in the end.. and that is fun

.. maybe berserk but with MR negates then.. so you loose some of your mages but the other still do something. And lets be fair with spells like MoD, SHimmering fields, MASTER ENSLAVE, undead mastery etc etc you can't really call any army killing spell overpowered... this one will hurt your army too remember.
btw your spell is also very powerfull and having that as midrange spell will still be.. game changing.. late game too, only now it starts earlier

This because you can easily make and army that takes full benefit of it (and it will be cheap) when opponents will only get 50% or less benefit... at least my idea was less about buffing yourself

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