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[Tt112233]

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[Accname]

One way is to make unholy frenzy a hero ability which requires level 6. That makes it automatically an ultimate ability and ultimates go through magic immunity.
If you dont want it to be shown as hero ability you can use a dummy ability which you give to your unit, and when this dummy abiltiy is being used you spawn a dummy caster with the real ultimate level hero ability unholy frenzy and force it to cast on the target.

 

[NotInTheFace]

Or make it a hero ability, set the required level > 1, then change it back to a unit ability. It will still be considered an ultimate.

 

[Tt112233]

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[NotInTheFace]

You can just make it a default ability if you change it back into a unit ability after setting the level requirement.

If you want a dummy to use a hero ability, you can add the ability with triggers and the dummy will be able to cast it without being a hero unit.