The Physicist: Three Astral Keys and Boss
Climb the Spire in Underpass, enter the Astral Plane and collect a key from each of its three branch islands. Open all three doors and use the mirror to reach the Physicist.
Quick answer
Climb the Spire in Underpass, enter the Astral Plane and collect a key from each of its three branch islands. Open all three doors and use the mirror to reach the Physicist.
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Spire boss route or Jesse Fault?
At the Spire fast-travel door, climb the red Astral surfaces using Shift. Defeat the enemies at the top to clear the route into the Astral Plane. This upward route is different from the Underpass Fault, which leads toward the orange-pipe island and Jesse.
Three branches, three keys
First face the locked doors. Use left, right and behind you as your reference: gravity changes make compass descriptions easy to misread. The keys can be collected in any order.
| Branch | Distinctive obstacle | Key landmark |
|---|---|---|
| Left / western route | Levers lower moving walls; avoid the exposed spike floor. Shift around the furniture-bearing pillar. | Cube on a table with chairs |
| Right / eastern route | Follow the furniture-bearing walls and stepped platforms, attaching after gravity changes. | Cube on a bed |
| Opposite doors / southern route | Use the tall side pillar to gain height before floating to the upper surface. | Cube on a desk |
A closed second door does not mean the first key failed: each door consumes its own key. After all three, continue to the mirror and hold its interaction.
Keep contact without chasing every projectile
The Physicist alternates low attack opportunities with fast movement into the air. Punish when it returns to reachable height; leave its pulse and explosion radius instead of forcing a stationary combo. When the arena shifts onward, follow the boss through the traversal route before resuming the fight. Use the final prompt after its health is exhausted.
Compare the reward in your Loadout
The reward offers Spike or Astral Rebuke. Read the displayed cost and effect before selecting, then test it with your current forms. A later Combat Ability reset can revisit that allocation; it is not a reason to spend a Reset Item on freely swappable weapon forms.
Before you leave
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Sources and scope
Written walkthroughs compared November 3, 2026. The diagrams explain the operation order; no site playthrough or performance benchmark is claimed.