The Dungeon Journal of the final boss in the dungeon Neltharion's Lair mentions that the skill Molten Crash will deal less damage and not knockback a tank that has used his Active Mitigation skill. As a Death Knight, I do not seem to have an ability that serves to nulify that effect, and after searching online I've seen mentions of Death Strike and Marrowrend being our Active Mitigation skills, but using both of them before being hit by Molten Crash still knocked me back. Am I doing something wrong or missing an important mechanic?
This is from an official statement from a game dev discussing bosses requiring active mitigation.
As a Death Knight your active mitigation skills are both, Death Strike and Marrowrend. You need to time them both correctly, though. Both skills give you a 3 seconds buff indicating that you just used an active mitigation (the buff is only visible when fighting bosses requiring active mitigation).
So it should be safe to assume that you need a good timing in order to counter Molten Crash.
I actually had some problems with that, too. With my mitigation up I got knocked back a couple of times as well. But as stated it should be safe when a mitigation is used 3 sec or less before Dargrul uses the knockback.
For Blood DK's the ability Marrowrend is labeled as "Active Mitigation".
Does it actually have a hidden 3 second grace window like Death Strike? or is this mis labeled? If a boss or mob is casting an ability that can be negated by AM and I use Marrowrend what happens?
Yes, it triggers the same 3sec window that Death Strike does. (That window is a visible buff, if the current encounter requires it).
This Post on IcyVeins contains information from a Blood DK who has researched this Molten Crash ability, the relevant section is copied below (emphasis added):
Your timing needs to be pretty good. So using Dragur as a benchmark I fought him a few times and played around. When he began casting molten crash I used death strike. By the time he finished the AM part of it expired but the blood shield did not. So, he smacked me, I went flying, and it did damage but not much. I had him smack me again with the same ability without using death strike at all, I went flying and it hurt bad. Time three, I waited until he was almost done casting molten crash, hit death strike, had the am part up when hit, and.... nothing happened really, didn't go flying, didn't take ugly damage, was fine.
So, the timing of the AM part is critical for boss debuff avoidance. I believe the blood shield part baked in is for it to work properly, specifically at higher lvls and that. You get the time where there are adds or boss aoe attack leftovers on you and you need the am part. Blood shield helps mitigate the aoe crap and add crap damage while the AM part seems to prioritize the boss debuff (not 100% on that). So, in those scenarios, I just use another death strike for the blood shield and then some other cd like AMS or DRW, and then cast death strike again right before boss attack to mitigate the debuff.