1. Don't use:
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Unit - A unit Begins channeling an ability
Instead use:
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Unit - A unit Starts the effect of an ability
2. Remove:
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(Unit-type of (Casting unit)) Equal to Priest
3. Leaks a group:
Unit Group - Pick every unit in (Units within 600.00 of (Position of (Target unit of ability being cast))) and do (Actions)
4. This here is sort of senseless as it doesn't serve any good purpose:
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Set special_effect_ID = (special_effect_ID + 1)
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Set special_effect[special_effect_ID] = (Last created special effect)
5. Although using the Wait here isn't much of a problem, if you know how to, avoid using it.
6. Neither MUI nor MPI.
To start off with, spell idea is very simple. However, work on fixing the just mentioned problems.
As a word of advice, use a special effect which you can destroy instantly and still get the effect. That way the spell would become instant (i.e. automatically MUI and wouldn't need the Wait anymore).