Just to make sure everyone knows what we're talking about, Withering Terror is the DoT put out by Kel'Sara that cannot be dispelled. The animation for Withering Terror looks like purple smoke hitting the ground around the player she targets. It has a small AoE so spreading out will make sure only one person gets it.
(There is another DoT during this phase applied by the adds that looks like a Sorcerer's Affliction DoT and is called Mass Affliction. This is not the DoT we are talking about. If you get Mass Affliction then you didn't kill the adds fast enough.)
Here is what I noticed from watching our kill/attempt videos:
- Kel'Sara casts Withering Terror for the first time 17 seconds after Ciphas dies.
- Withering Terror lasts 25 seconds.
- Kel'Sara will not cast Withering Terror again until 11 seconds (or more) after the previous Withering Terror drops off.
- She casts it on people standing at range, not on people in melee range.
So basically you have 17 seconds into phase 3 and then Withering Terror goes out. Then every 36 to 40 seconds after that she will cast it until she is dead. To mitigate this mechanic I recommend spreading out when no one has Withering Terror and once it gets applied to someone you can move closer together for AoE healing.