Progress bar indicating Midnight's Health Bar to show her progress of healing up during Riderless phase on Attumen fight.
The dismount triggers when she reaches 50% health (but can be delayed and you can damage her more to get her to lower health) where after she gets a buff making her immune and makes her heal 2% HP every second (heal reduction/mortal wounds effects are still active on her however, until they fall off, making her heal less initially during dismount, prolonging Attumen phase).
So baseline if the dismount is exactly at 50% and no heal reduction is applied, it takes 25s before he jumps back on her, but if you get her lower than 50% and/or her heal is slowed by heal reduction, it can take longer.
If Attumen is casting when she reaches 100%, his cast is cancelled for the remount, however, if she's casting (her Mezair frontals) that cast will first finish and then remount happens.
This bar helps you see her health filling up during dismount phase + gives you a "timer" on the right that's basically an indication of how many 1s ticks of 2% HP heals she has left till she reaches 100% health. This timer is thus not accurate if she has heal reduction effects initially, but does/should get accurate later part of the dismounted phase as those effects fall off (and she shouldn't get new ones as she's immune).
Style based on M+ S4 Dungeon Casts by alethasis, here: https://wago.io/mZGjMqPJv
If you put this in that pack in the Karazhan Lower group and place it above all the other Attumen/Midnight WAs, this bar should always be on top of the others and allows you to easier see if she's doing a frontal (so even if this bar fills up to 100%, if right under it you see Midnight's Mezair cast you know that'll finish first before remount). If alethasis chooses to include this bar in their pack, I can remove/hide this away