Emerald Nightmare Mythic difficulty - Il'gynoth encounter:
-Inconsistent spawnrate of tentacles. Meaning you get really low amount of adds during the fight.
-The fight isn't treated like a boss fight, meaning you can ress and run back to the fight.
-When the first time Eye of Il'gynoth dies and exposes its heart, while inside you can't attack it by direct attacks, only aoe attacks do damage to it, also while standing outside of the tree, you can target the heart with ranged direct attacks.
Here is the full recording of the fight: