Activation is what starts the fight. The arena controls who sees the boss bar and gets the player support you've enabled. You can use the same box for both, or give them separate areas.

/boss select activation my_boss
/boss select arena my_boss

Run the selector you need, then left-click for corner one and right-click for corner two. The arrow is automatically selected in your hotbar. It belongs to your active selector session; ordinary arrows still work normally.

To use the same box for both settings:

/boss select activation my_boss true

For activation selection, sneak + right-click switches between yellow AREA and red REDSTONE mode. Combined activation/arena selection stays in AREA mode. To stop selecting:

/boss select cancel

Each box axis is limited to 256 blocks. Invalid or oversized boxes in hand-edited files are ignored. Holding the selector button does not repeatedly select or start mining after completion; a fresh click restores normal input.

Spawning while players are already inside does not activate the boss. Reloading changed triggers resets which players count as already inside.

Dormant activation is checked every 10 game ticks, about half a second at normal speed. Keep redstone pulses on for at least 10 ticks so the check can observe the rising edge.

Choose the shape in the editor, or edit these fields in your template. This example only shows the fields you need:

{
  "activation": {
    "type": "player_enter_radius",
    "radius": 32.0
  },
  "boss_bar": {
    "visibility_area": {
      "shape": "radius",
      "radius": 32.0,
      "dimension": ""
    }
  }
}

Both radii are full 3D distances, including height, bounded to 1–256 blocks.

• The activation sphere stays centered on the emergence position.
• A radius boss-bar arena follows the Warden and shares membership with player buffs.
• The selection arrow saves box mode. Use the editor or configuration for radius mode.

Save file changes and run /boss reload to apply them.

A newly spawned boss waits invisibly below its saved emergence position, with AI disabled. Its hidden depth scales with size: 5 blocks at scale 1, 10 at scale 2, and 20 at scale 4.

Activation moves it back to the saved position before Minecraft's emergence sequence. Dormant bosses have no boss bar. Their configured underground Darkness can still affect nearby players from the actual hidden position.

Once you activate the boss, players in the arena receive the support you enabled, even during emergence or if they arrive late. Encounter music also starts on activation. Darkness itself has no aura during emergence.

Your boss-bar area also sets the boundaries for player support and encounter music. It helps determine who the boss can target, with follow range and beam range adding their own limits.

Test the edges and height of your region, not just its outline on the floor. With a sphere, a player high above the fight can be outside even if their map position looks right.