UNLOCKING MYTHIC PATH:
AEON: Pick up the Nehyndrian Chisel (interact spot in the Market Square, near Terendelev's huge blood stain), retain the Aeon after the book event, select the Aeon-specific choice when purifying the Wardstone.
AZATA: Side with Ramien or reconcile him with Hulrun, find the three Acolytes of Desna in Kenabres (Time-sensitive: Hulrun will kill him if he's still alive and not pacified after the Defender's Heart quest) and sing with them when they ask you.
LICH: Explore Leper's Smile underground and take the wand deep into the cave. At the Ruined Chapel, a secret wall will open after entering the temple on the right. Accept Zacharius' offer to unlock the path.
TRICKSTER: Fool Chaleb into bombing his own allies by picking the Chaotic option during his dialogue (can be done either before or after the tavern siege), then collect the three clues about Nurah being a spy (Perception check when asking about her past, find her throwing alchemist's fire during the gargoyle attack on the camp (immediately northwest from the commander's tent), find the bottle of Vescavor pheromones after Leper's Smile) and confront her in the camp, then ask her to play a joke on the demons with you.
DRAGON: Keep Terendelev's Scales and Fang (the Fang is inside Leper's Smile) and have the Storyteller tell you their story. Find Hal in a ruined temple, show mercy to the surviving cultists, and accept his help. Do his quest (keep the scales and fang in your inventory.
SWARM: Pick up the vescavor pheromones in Leper's Smile and use it in the attack on Drezen (must NOT have Galfrey in your camp, she will forbid it.) After beating Xanthir Vang, ask him about his transformation and you will gain access to his notes, this will unlock the decrees to start sacrificing your soldiers to eventually unlock Swarm.