Add the bundle to /bundles, add to ranvier.json, and then set the behavior in any area's manifest.yml.

title: Northern Reach
behaviors:
  area-respawn:
    spawner: true
    scaleMobLevelOnAxis: y
    flipScale: false
    mobLevels: [0,30] 
    mobToRoomRatio: 0.35
    sourceNpcsFromAreas: ["area1","otherarea"] 

spawner (boolean) - enable or disable entire behavior for this area

scaleMobLevelOnAxis (string) - use x or y. Sets the axis to scale difficulty of mob spawns along.

flipScale (boolean) - by default, lower coordinates on the axis correspond to lower mob levels, set this to true to cause the reverse.

mobLevels (array) - takes an array of two values, the first is the minimap, and 2nd is max. These values get blended a bit ~15% each direction in the code, so it's not a hard boundary.

mobToRoomRatio (float) - uses the number of rooms in the area to determine the amount of mobs to keep spawned

sourceNpcsFromAreas (array) - which areas to source mobs from, has no default

This does not randomly generate mobs or dynamically adjust their level. All it does is pull a group of npc definitions from a list of areas, and randomly choose a room with the spawner area, calculate what the level difficulty for that coordinate should be, expand it out about 20% on each side, and see if any of the npc definitions fall within that range.

If one can't be found, it doesn't spawn for that room. So you need a sufficiently full list of npcs with a range of levels in order to appropriately spawn an area.