Climbing the Spire
The Underpass opens in the main mission Recursions and Iterations. You need Shift from the Incursion Fault; see abilities if you skipped it.
- 1. Unfog the Underpass map and zoom in until the label The Spire appears.
- 2. Start the quest by inspecting the eye icon from the north side of the Spire, or simply start climbing.
- 3. Stand left of the fast travel door and use Shift to walk up the red blocks on the Spire’s side.
- 4. Clear the enemies at the top. The red blockage turns into white light: walk through it into the Astral Plane.
- 5. Climb, jump to the side of the floating path and Shift onto the wall straight away, then walk up to the locked door.
All 3 Astral Key locations
PowerPyx places one key on each of the three floating islands around the locked door. You can take them in any order:
| Key | Island (facing the door) | Route notes (Whisper of the House) |
|---|---|---|
| 1 | Directly across, the far end from where you climbed up | Through the Astral spike-trap boxes; take the right path and keep straight |
| 2 | To the right of the door | Jump the gap, pass the spikes; at the chair and TV, Shift down twice and jump across |
| 3 | To the left of the door | Skip the first floating platform, take the second and follow the box that moves away as you approach |
Whisper of the House describes three locked doors opened one after another, each with its own key; PowerPyx describes one door opened with all three. Either way you need all three keys. The route notes come from Whisper of the House, so the key numbering may not match PowerPyx exactly. Do not waste time in the Research Site’s yellow-tape branches; the keys are not there.
How to beat the Physicist
Top up health before the last door: the key routes are full of fights. If ordinary enemies already take too long to kill, leave, spend Talent Points or upgrade your Aberrant first.
- It charges area explosions and throws debris volleys. Circle wide rather than backing straight up.
- Jump the ground sweep, and kill summoned minions before they stack up.
- KeenGamer says it punishes greedy combos harder than any earlier boss. Take two or three hits per opening, then reposition.
- Build Falter and take every Execution prompt. When its health hits zero, Dylan is pulled back to the Gap for the reward.
Spike or Astral Rebuke
| Spike | Astral Rebuke | |
|---|---|---|
| What it does | Plants an Astral Spike that knocks enemies down | A homing spike erupts under the target and launches it |
| Power (Destructoid) | 305 | 155 |
| Catch | Value depends on keeping it in play | Needs a grounded target; weak against flyers |
| Suits | Summon or crowd-control builds | Straightforward melee and air combos |
Both apply Astralized, which makes enemies build other status effects faster — a good partner for Ignite or Inferno. A Reset Item at the mural in the Gap swaps the pick later.
Rewards
- Spike or Astral Rebuke (pick one)
- Untapped Golden Earrings Artifact (+20% Falter, −30% damage; see Artifacts)
- 1 Reset Item and 3,000 Source
While you are in the Underpass, grab the bulbs for Maintenance Protocol and the zone’s taxi and dog toy. Every other Resonant is on the bosses page.
Underpass Resonant FAQ
Where is the Underpass Resonant? At the top of the Spire in the Underpass. Climb it with Shift and walk through the white light at the top into the Astral Plane, where the Physicist waits.
Where are the three Astral Keys? On three floating islands around the locked door: one straight across, one to the right and one to the left (PowerPyx). None of them are marked on the minimap.
Spike or Astral Rebuke? Spike plants an Astral Spike that knocks enemies down. Astral Rebuke launches a grounded target into the air for a combo. Both apply Astralized, and a Reset Item lets you swap later.
What do you get for beating the Physicist? PowerPyx lists the Spike or Astral Rebuke pick, the Untapped Golden Earrings Artifact, one Reset Item and 3,000 Source.
Sources
Related guides
DiagramGuidesControl Resonant Bosses: All Resonant Locations and FightsRead guide 
DiagramGuidesControl Resonant Abilities: Every Power and How to Unlock ItRead guide 
DiagramGuidesControl Resonant Ignite or Inferno: Which Should You Pick?Read guide 
DiagramGuidesControl Resonant Maintenance Protocol Walkthrough: Every Bulb FixedRead guide