Control Resonant: The Underpass Resonant Quest Walkthrough (The Physicist Boss)

TL;DR:

• The Underpass Resonant is the 4th main mission in the Defeating Resonants questline.
• Unlocks after reaching the Underpass during Recursions and Iterations — activate The Spire fast travel door first.
• Use Gravity Shift to climb the red blocks on the side of The Spire. Clear enemies at the top to open the white light gateway into the Astral Plane.
• Collect all three Astral Keys from the floating islands — one directly across, one to the right, one to the left of the locked door.
• Open all three doors and fight The Physicist. Keep moving, stay out of its AoE blasts, and clear summoned minions when they appear.
• After the fight, choose between Spike (single-target knockdown summon) or Astral Rebuke (homing spike that launches groups).
• Rewards: Spike or Astral Rebuke ability, Untapped Golden Earrings artifact, 1x Reset Item, 3,000 Source.

The Underpass Resonant is the boss fight you see hinted at from the moment you arrive in the Underpass — that tall, glowing Spire visible from the main path during Recursions and Iterations. Getting to the boss requires climbing above the main quest route, entering the Astral Plane, and tracking down three keys that are not marked on the minimap. This guide covers every step, all three key locations, and how to beat The Physicist cleanly.

The Underpass Resonant Quest Overview

Main Story Chain: Defeating Resonants

Quest Number: 4th Resonant mission

Starting Location: The Underpass Zone — The Spire

Requirement: Reach the Underpass Zone as part of the main quest Recursions and Iterations

Rewards: Spike or Astral Rebuke (Combat Ability), Untapped Golden Earrings (Artifact Recipe), 1x Reset Item, 3,000 Source

Quest Description: Dylan finds a spire-like structure in the Underpass made of Astral material and goes to investigate.

Boss Name: The Physicist

Before taking on this boss, make sure your build is in good shape. Our how to respec guide covers reallocating Talent Points, and our all Power Upgrade chest locations guide will help you top up your health and ability stats before the fight.

The Underpass Resonant Quest Objectives

1. See Where the Spire Leads
2. Investigate the Astral Plane
3. Kill the Astral Plane Resonant

How to Start The Underpass Resonant Quest

You reach the Underpass as part of the main quest Recursions and Iterations. Once you unfog the Underpass map, you will see “The Spire” text when zooming in. The Spire is visible from the main research site path as a tall, glowing structure.

There are two ways to start the quest:

• Method 1: Stand north of The Spire and look for an eye icon on screen (L1/LB). Inspect it to add the quest to your log.
• Method 2: Simply start climbing The Spire. The quest logs automatically once you begin ascending.

The Spire fast travel door is right at the base. Activate it if you have not already — it is the quickest way back if you need to leave and upgrade before attempting the boss.

This quest becomes available during Recursions and Iterations but you do not have to do it immediately. You can finish the main quest first and return here. If you want the full picture of what Recursions and Iterations involves before branching off to The Spire, our Recursions and Iterations full walkthrough covers every step.

Step 1 — Climb The Spire

The Underpass Resonant Quest Control Resonant

To the left of The Spire fast travel door, look for a series of red blocks running up the side of The Spire. Use your Gravity Shift ability to walk up these blocks. They function as a wall-run surface — activate Shift and move into the red blocks to stick and climb.

Keep climbing until you reach the top of The Spire. The environment transitions as you get higher — you are leaving the regular Underpass geometry and approaching the Astral boundary.

Step 2 — Open the Gateway into the Astral Plane

At the top of The Spire, a group of Astral enemies is blocking a red energy barrier. You need to defeat all of them to dissolve it.

• Clear every enemy in the area.
• Once the last one drops, the red blockage transforms into a bright white light gateway.
• Walk through the gateway to enter the Astral Plane.

Tip: The enemies here can hit hard. Do not rush in — use Telekinesis throws and ranged weapon fire from a distance first, then close in. Make sure you are at or near full health before going through the white light, because the Astral Plane section begins immediately on the other side.

Step 3 — Navigate the Astral Plane

Once through the gateway, you are in the Astral Plane — a void of floating white platforms, apartment fragments, and shifted geometry. This section is the route between the gateway and the boss door.

Climb up the first platform in front of you. Use a double dash jump to cross wider gaps. When you are close to a wall and would otherwise fall, activate Gravity Shift to latch onto the wall surface and walk up it rather than dropping. This is how you navigate the more vertical sections of the Astral Plane.

Follow the path upward until you reach the end — a pair of large locked double doors. This is where the key hunt begins. You need three Astral Keys to open all three locks and reach The Physicist.

Step 4 — All Three Astral Key Locations

The keys are not marked on your minimap. They sit on three separate floating islands around the locked door. You can collect them in any order.

Astral Key 1 — Directly Across from the Locked Door

Turn around from the locked door and look back the way you came. The first island is directly across — the same side you climbed up from, but at the far end away from the doors. It is visible from the door platform. Jump across and follow the island to its end to find the key. Look for a dining area or table surface in the fragment — the key sits on or near it.

Astral Key 2 — Island to the Right of the Door

From the locked door, look right. The second floating island is off to the right side of the door platform. Jump across and navigate to its far end. This fragment contains a room with a bed inside an iron or metal structure. The key is at the end of this island path.

Astral Key 3 — Island to the Left of the Door

From the locked door, look left. The third island is off to the left. Jump across and follow it to the end. This fragment features an inverted corridor section — use Gravity Shift to walk along the ceiling or walls to navigate through it. The key is on a desk surface at the far end.

Once you have all three keys, return to the locked door. Use them to open all three locks. The doors unseal and the path to The Physicist opens ahead.

Step 5 — How to Beat The Physicist Boss in Control Resonant

The Physicist is a large, sphere-like Astral entity. It is the Underpass Resonant — the boss the Quest Log refers to throughout this mission. The fight takes place across a multi-section arena that shifts as the encounter progresses. There are no health items available inside boss arenas, so execute lesser enemies during the fight to recover health. Do not kill them all at once — leave a few alive as a healing resource.

Phase 1 — The Chase and Opening Attacks

The Physicist opens by chasing Dylan across the arena and charging up a large area-of-effect explosion when it gets close. Do not stand still. Keep running until it stops repeating the blast cycle, then turn and attack during the brief recovery window.

Its other main attack patterns in this phase:

• Debris volleys: The Physicist rises into the air and throws three waves of debris outward. Sprint in a wide circle around the arena to outrun every wave rather than trying to dodge through them.
• Ground sweep: When it drops back to the ground, it sweeps a limb around its base in a low arc. Jump to avoid this — do not try to roll or sidestep it. Attack from the air while you are above the sweep.

Attack during the gaps between these patterns. Short burst combos followed by repositioning work better than committing to long attack strings while the arena geometry is shifting around you.

Phase 2 — Arena Shift and Minion Summons

After depleting the first health bar section, The Physicist moves to a second arena zone. Chase it using your traversal abilities — jump and glide to follow it across the gap. The fight continues in the new space with the same attack patterns at higher aggression.

Later in this phase, The Physicist breaks apart into several smaller enemies. These are slower and less powerful but they serve as distractions. Kill all the smaller enemies first — this forces The Physicist to rebuild itself whole and temporarily reduces incoming threat. Once it rebuilds, resume attacking it directly.

Phase 3 — Final Push

After the second health tick depletes, The Physicist flies to a third section of the arena. Chase it down again. The attack patterns do not change in this phase — it uses the same AoE blasts, debris volleys, and sweep attacks, but with shorter downtime between them.

Stay patient. This phase rewards consistent pressure during the attack windows over trying to maximise damage while the arena is unsafe. Keep moving, stay above ground sweeps, and circle during debris volleys.

Once the final health bar is emptied, an execution prompt appears on screen. Use it to falter The Physicist and finish it off. The Astral breach sequence ends and Dylan is taken to the Gap for the reward selection.

Physicist Boss Fight Tips

• Never attack through a full combo while the arena is shifting. Commit short bursts and reposition after each window closes.
• Save some minions for health recovery. You cannot use health items in boss arenas. Execute weaker enemies to restore HP mid-fight.
• Jump during ground sweeps, do not roll. The sweep catches rolls. Jumping clears it cleanly and puts you in position to attack from the air.
• Circle the arena during debris volleys. Wide sprinting circles outrun the waves more reliably than trying to judge individual dodges.
• Kill the Phase 2 minions immediately. They are slow but they block your sightlines and eat your ranged attacks. Clear them first every time they appear.
• If your damage is very low, leave and come back. You can exit the Resonant route at any point. Spend Talent Points, upgrade the Aberrant weapon, and return. The three-key section does not reset.

If combat is proving too difficult, our Assist Mode settings guide covers all the options available including immortality, which does not disable trophies.

Step 6 — Spike or Astral Rebuke: Which Ability Should You Choose?

After defeating The Physicist, the game presents you with a choice between two Combat Abilities. Both abilities build the Astralized status effect on enemies, which makes them more vulnerable to all other status effects including Burn and Infected. Both cost 250 power to use.

Spike

Spike summons a small ball of Astral energy that acts as an autonomous ally, attacking nearby enemies. It is a single-target, directed summon that knocks down the chosen enemy. Good for players who like autonomous combat pressure and summon-style builds.

Astral Rebuke

Astral Rebuke is a homing spike attack that launches enemies into the air with the Lifted status effect, similar to the Physicist’s own travelling spike move. It hits harder than Spike, covers groups rather than individuals, and is more reliable in cluttered fights because of the homing property.

Which Should You Pick?

For most players, Astral Rebuke is the stronger choice. The homing property makes it reliable even on fast-moving targets, the Lifted effect sets up aerial combos, and it handles groups better than the single-target Spike. Spike is a better pick if you prefer a build that lets you focus on positioning while a summon handles damage.

You can change your ability selection later at any time by visiting The Gap and interacting with the Mural. The choice is not permanent, so pick whichever one suits your current build and adjust later if needed.

After choosing, activate the shrine in the dark stone chamber to open the Artifact Formation menu and claim the Untapped Golden Earrings recipe.

The Underpass Resonant Rewards

• Spike or Astral Rebuke — your chosen Combat Ability (both apply Astralized)
• Untapped Golden Earrings — Artifact Recipe (claim from the shrine after ability selection)
• 1x Reset Item
• 3,000 Source

Other Guides for This Point in Control Resonant

If you are working through the Underpass region at the same time, these are worth having open:

• Recursions and Iterations main quest walkthrough
• The Subway Fault — how to unlock the Reach ability
• Maintenance Protocol quest — all three bulb locations
• How to restore power in The House in Distress
• How to reach the Unknown Zone
• Crossing the Threshold walkthrough

For skin rewards and cosmetics you may have missed, check our Control Resonant skin codes guide and our pre-order and Deluxe Edition bonuses breakdown.

Frequently Asked Questions

Control Resonant is available on the official Control website, PlayStation Store, Xbox Store, Steam, and Epic Games Store.