Primrose's Passage was an unimplemented area unlocked in 1.54 and renamed The Temple of the Hidden Sun. The Passage / Temple is a secret area like the Secret Passage from TQ:IT.
Access to the Primrose Passage is accomplished by completing a Hidden Quest. The quest involves finding a hidden items (Spell of Pieces) in each of the first four acts. Completing the hidden quest is rewarded with The Primrose, a special key just like The Overlord.
The entrance to Primrose's Passage is a semi-hidden cave entrance located on the far east side of the Deep Forest.
The Passage / Temple has a Majestic Chest that has a high chance of dropping rare Set items and Legendary Items. Two Golden Orbs also drop from Bosses inside the Passage / Temple.
After completing a special, very secret, bonus quest you get access to the Temple of the Hidden Sun, previously referred to as The Primrose Passage. The Temple is populated with Laser Raptors, Druoids, and Skeletal Nymphs. Just like the Secret Passage from IT, the Temple of the Hidden Sun is populated with bosses named after people that worked on the game.
Two bosses, Jimmy and Erik, can most easily be defeated with the help of the Secret Ally. Robin and Erik drop Golden Orbs. The Golden Orbs can only be opened after clearing all the monsters and Bosses. However, the Secret Ally prevents the Golden Orbs from being unlocked. Disband the Secret Ally in order to loot the Golden Orbs.
The super Majestic Chest, the Dark Chest of Wonders, can be opened after killing all the bosses. The Secret Ally does not interfere with opening the Dark Chest of Wonders.