They spawn roughly every 2 weeks.
You need 15 heroic bosses killed to unlock them.
The question is : Do you need to kill 15 heroic bosses every 2 weeks or is it a onetime thing?
I failed an 82% one this week, I didn't have a follower that could counter Magic Debuff.
I will for my next one though.
I've determined there is no 99%, there is either 50% or 100%. Everything else is a lie.
How do people not 100% every highmaul yet?! ... anyway
You don't need to do that mission asap. You keep the Highmaul missions in your garrison for 13days 23 hours. Those missions have a 14day CD(then randomly appear after off CD) from the instant they go into your garrison report. Not from when you start them.
Protip to all those who want to reduce their failure chance: Blook is your best friend.
Trust in Blook. Put your faith in Blook.
Rely on Trait stacking rather than Ability countering. If you can get 4-5 traits in one mission that increase mission success chance, you only need to counter half, or even less than half of the abilities in a Mission.
Blook is indeed good with his unique trait, problem I have with him is there is no trait that increases % chance with Blooks race, other wise I'd keep him. I only activate followers of Alliance races to benefit in Mission success chance through traits