Looking at the Mythic Plus affixes this week, there are
TYRANNICAL BOLSTERING EXPLOSIVE INFESTED
The explosive affix is stated to be:
Creatures have a chance to summon an Explosive Orb at a nearby location that will explode, inflicting damage 50% of the player’s maximum health as damage.
Note: An explosive orb will spawn with a 6 second cast. If the cast goes off the entire party will take 50% of their max hp in damage. They are AOE immune so you can't cleave them down! They can also spawn on boss fights.
The note mentions that they are AoE immune and cannot be cleaved down. Which spells count as AoE? Would a spell such as Chain Lightning be considered AoE?