The mass spawner is the most basic of the three large machines that allow world owners to configure various aspects of mob spawning on their worlds. You can find this item in the Engineering section of inventory. One of the largest items in the game, the mass spawner is five blocks wide and four blocks tall, is self-protecting, and has no collision. Note that you can only place this item in a world that you own.
How to Obtain
You can purchase this item from the shop or trade for it on a market world.
How to Use
Place the mass spawner where you want it, then tap or click on it. This will bring up a list of options.
With the basic mass spawner, you have only three options: peaceful, normal, and dangerous.
Peaceful: stops all hostile mobs from spawning, both from maws and in open areas. Non-hostile mobs will still spawn as usual.
Normal: causes mobs to spawn the way they always do.
Dangerous: causes more hostile mobs to spawn, both from maws and in open areas.
This option lets you relocate the machine. Once you choose this option, you can then place a beacon (the thing that goes with a regular target teleporter) where you want the machine to move to. Note that the machine will position itself so that its bottom left block is where the bottom block of the beacon is. The beacon will be consumed in the process.
Tragically, as of this writing (February 2016), this command does not work. There is no way to return a machine to your inventory at this time
If you decide you don't want a machine on your world after all, you can destroy it by using the "move" command to relocate it so its bottom left block lies directly on top of an open culvert. First configure the machine to "peaceful," then move it to the open culvert, then configure it to "dangerous." Be aware that this will completely destroy the machine, and you will not be reimbursed for whatever crowns (if any) you spent on it.
It is possible to place more than one version of the same machine on your world ... but why would you want to?
Mass spawners were added in July 2015 with the 1.29.12 release.