I've tried to make this work, but I simply can't do it on my own. I want a weakaura that shows me the time until Prydaz activates as a simple timer.
The buff, in case you don't know, is called "Xavaric's Magnum Opus"
The biggest problem with this is that the item itself doesn't show the cooldown, it's only a buff which when gone, doesn't have a replacement that you can track. My attempts so far have mostly resulted in either the timer disappearing with the buff or only starting when it goes away, both of which are fairly inaccurate for tracking it.
I would create a custom timer trigger in Trigger > Event > Combat Log > Spell > Aura Applied, with destination player, Spell Name "Xavaric's Magnum Opus", then you set it to count down from 30, create a bar to watch it, etc.
If the shield is not consumed it will show up as SPELL_AURA_REFRESH instead of SPELL_AURA_APPLIED.
Add a second trigger with Message Suffix set to Aura Refresh and set "Required For Activation" to Any Trigger.
Now it shows up, but it doesn't display the timer, it's just an empty bar (just the background) when set to a progress bar. If I have it set to text it doesn't show up at all, and even as a progress bar, I'm not sure how to get the timer showing. Would you have an idea on how to do that?