Short answer: In Deltarune Chapter 5, you “unseal” the Mysterious Structure by first unlocking the Petal Feather ability (via Aqua), then using it to climb the giant vines and reach the high platform where the structure’s power activates—this removes the seal and lets you proceed.

What “the structure” is and why it’s sealed

Early in the Flower Castle area, you’ll see a large, inactive Mysterious Structure next to a wall of thick vines. Interacting with it does nothing at first because:

  • The structure needs a special power to activate.
  • That power is tied to a new movement ability you don’t have yet.

This is an intentional progression gate: the game wants you to meet a specific NPC and get an upgrade before you can use the structure.

Step‑by‑step: how to unseal / activate the Mysterious Structure

Follow these steps in order:

  1. Progress until you meet Aqua
    • Continue through the Garden / Flower Castle routes until you encounter Aqua , a key NPC tied to the chapter’s new mechanics.
  1. Get the Petal Feather ability
    • After the relevant events with Aqua, you obtain the Petal Feather.
    • This ability lets your party climb or glide up certain vine walls and vertical paths that were previously unreachable.
  2. Return to the giant vines near the Mysterious Structure
    • Backtrack to the area with the big vine wall right next to the sealed Mysterious Structure.
    • Use the Petal Feather on the vines to climb upward.
  3. Reach the upper platform and activate the structure
    • At the top of the vines, you’ll find a platform or interactable point linked to the structure.
    • Interact there to “activate” or “power” the Mysterious Structure.
    • Once activated, the seal is removed: new paths open, and progression continues.

That activation is effectively what “unsealing the structure” means in Chapter 5: you’re giving it the power it needs so it stops being a dead end and starts functioning as part of the level.

Common sticking points

  • “It still does nothing when I touch it”
    • You almost certainly don’t have the Petal Feather yet. Keep following the main path until Aqua’s sequence is complete.
  • “I have the feather but can’t reach it”
    • Make sure you’re using the feather on the correct set of vines—usually the ones directly beside or visually connected to the Mysterious Structure.
  • “I activated it but nothing changed”
    • After activation, check for:
      • Newly opened doors or paths nearby.
      • Dialogue or cutscenes that trigger once you leave and re‑enter the area.

TL;DR

  • You can’t unseal the Mysterious Structure at the start.
  • Meet Aqua , get the Petal Feather , climb the giant vines next to the structure, and activate it from the upper platform.
  • That removes the seal and opens the way forward.

Information gathered from public forums or data available on the internet and portrayed here.