Borderlands 4: Pyroclast Grotto Occupied Sector Full Walkthrough

Pyroclast Grotto is one of two Occupied Sectors added with Providence Island, and it is the shortest path to unlocking Blackgrin as a farmable boss. The run is a single linear cave route built around lava, ziplines, and a handful of shootable switches, ending in an arena fight against a level 71 elite creature.

Quick answer: Travel to Pyroclast Grotto in the Pyroclast Field region of Providence Island, shoot the glowing blue Polyclusters to open the Tediore security door, follow the cave route through two ziplines and a jump pad to the arena, then kill Blackgrin to trigger the “Occupied Sector Complete” notification.

Pyroclast Grotto location and objectives

The sector sits inside the Pyroclast Field region on Providence Island, the map introduced with the FL4K and the Last Resort story pack. Open your map, look for the Occupied Sector icon in Pyroclast Field, and set the waypoint. The activity tracks three objectives in order, and each one only appears once the previous one is done.

Objective	What it requires
Open the door	Destroy the blue Polyclusters on the Tediore security door at the cave mouth.
Explore the sector	Traverse the cavern route using ziplines, a jump pad, and shootable switches.
Kill the boss	Defeat Blackgrin in the arena at the end of the route.

Note: Pyroclast Grotto is the second of the two Occupied Sectors on the island. The other is on Akela Island and unlocks Drilla Killa instead.

Full route through the grotto

Walk up to the Tediore security door blocking the cave entrance. The door is held shut by glowing blue Polyclusters, so shoot each one until they break. The shield drops and the first objective clears immediately.

Head into the dark tunnels and follow the passage inward. There is some light platforming here as you climb toward the facility built into the rock, but the path does not branch in any meaningful way. Keep following the blue objective marker if you lose your bearings.

Occupied Sector objective panel tracked on the HUD while moving through the grotto tunnels — The Occupied Sector panel tracks your progress as you push through the tunnels. Image: Gearbox Software

The tunnels open into a wide cavern with lava running through it. Look for the generator switch on the far side and shoot it to power the zipline, then ride the line across the gap.

On the other side, continue through the tunnel until you find the jump pad. Use it to reach the higher scaffolding, then cross the narrow metal bridge that runs over the lava below.

Rocky cave section of Pyroclast Grotto with the Explore the sector objective active — The “Explore the sector” objective stays active through the rocky middle stretch of the cave. Image: Gearbox Software

Drop down from the bridge and look for the electrical switch on the wall. Shoot it to unlock the doorway, then move through the rocky passage behind it.

Crosshair lined up on a V-marked interactive device inside Pyroclast Grotto — Interactive devices marked with the V prompt gate the route forward. Image: Gearbox Software

Climb the stairs into the industrial part of the facility. Tediore security forces defend this area, so clear them out while using the large pipes and containers as cover.

Aiming down sights at a Tediore enemy with the reload prompt on screen — Tediore security forces hold the upper facility before the arena. Image: Gearbox Software

Before moving on, check the high ledge in this area for a large red Tediore chest. It can contain legendary weapons and shields, and it is the only guaranteed chest on the route, so it is worth the detour.

Take the second zipline, climb the run of stairs at the far end, and slide down the long chute. That drop puts you directly into the boss arena and switches the objective to “Kill the boss.”

Blackgrin boss fight

Blackgrin is a huge reptilian creature that fights at level 71 and mixes physical charges with elemental attacks. Elemental weapons chew through the health bar fastest, and the arena is open enough that you can keep moving and reposition while your shields recharge rather than trading damage head-on.

Fighting the giant reptilian boss Blackgrin with the Kill the boss objective on screen — Blackgrin, the level 71 elite that ends the Pyroclast Grotto run. Image: Gearbox Software

When it dies, the boss drops Eridium, cash, and high-tier loot on the arena floor. Clearing the sector also adds Blackgrin to the roster of bosses you can return to and farm for dedicated drops.

How to confirm the sector is cleared

The “Occupied Sector Complete” banner appears on screen the moment Blackgrin goes down. Open the map afterward and the Pyroclast Grotto marker in Pyroclast Field will show as completed. If you are still seeing an active objective, one of two things is usually the cause.

The entrance door stays sealed until every Polycluster on it has been destroyed.
The zipline and the blocked doorway only activate after you shoot their respective switches, so backtrack and check for one you skipped.

Sweep the arena for loot beams before you fast travel out, since the legendary drops land where the boss falls and are easy to leave behind once the completion banner pulls your attention to the map.