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.
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory