The Goblin Balloon is available after players reach level 10.  The balloon appears by collecting Goblin Crates from Wanted Missions.

The items which you receive from the Goblin Crates are not added to your inventory, but rather they fill a slot on your Goblin Balloon. Once you have filled all the Goblin Balloon slots with Goblin Crates you can either claim the displayed items along with a Mystery Crate by purchasing the offer or you can recycle its content.

Recycling the content gives you a small amount of loot and sends the Goblin Balloon away until more Goblin Crates are found.

It is possible to claim the Goblin Balloon loot before all the slots are filled, but in that case, the Mystery Crate is not included in the offer. Please note that the only way to reveal the contents of a Mystery Crate is to purchase the items stored in a full Goblin Balloon.