A Rainbow Key is the item named for entering a dream realm. Steam's Key to the Dream achievement specifies spending one to enter. That answers the key-identity question; it does not establish where the key drops or where the entrance is located.
Check the entry item
Read the key name shown by the entry prompt and your inventory before spending anything. The official Key to the Dream achievement names a Rainbow Key. Do not assume that an item called a boss key is interchangeable because both names contain “key.”
This source does not provide a farming location, entry level, guaranteed reward or key drop probability. If you do not have the requested item, this page cannot supply a verified farm route; check the source shown in your current game rather than following an unsupported drop claim.
Dream realm and Mythic Rift are separate references
Steam also lists Into the Endless Realm for entering the Mythic Rift. The achievements use different names and objectives. That is not enough evidence to say that the Rift uses a Rainbow Key, shares an entrance or unlocks at the same level.
The official store description describes Mythic Realms as timed challenges with increasing difficulty and rewards. It does not resolve every naming or unlock detail between that description and the achievements.
Choose your next task
If you are looking for a regular named boss, use the boss-to-region lookup. If you are checking progression goals before using scarce items, read the equipment milestones. Keep the key requirement separate from unverified reward and farming advice.
Steam / Windows · Checked September 30, 2026 · Written and source-reviewed by Deskrawl Guide Hub; not hands-on tested.