After completing the 25. No Sadness Allowed side quest in Trinity Trigger, speak with Isaac in Festibane to begin the 26. A Waking Dream quest. For this quest, you’ll need to find an Awakening Seed in the Festibane Trail: South. Head to the eastern side of the area to trigger a cutscene with 3 yellow mushrooms.
Defeat the mushrooms then loot the shiny item by the blue tree. Afterwards, return to Isaac to complete the quest. As a reward, you’ll receive an Energy Booster IV.