Collections are sets of items united by similar properties or purposes. You can find a list of all collections by tapping on the “Collections” icon on the right side of the bottom row on the world map view. There, in the bottom left you can switch between viewing all collections and the ones that are ready to be completed.
Collections consist of 4 items, and at least one of each of these items is required to complete it. Additionally, collections require “charges” – more common items that are used to bind the elements of the collection together. You need to have a sufficient number of all of the required charges. The requirements for each collection are shown in the collections screen – from left to right 4 elements are shown, then a square with up to 4 panels showing the required charges. The item on the right is the one you will receive for completing the collection.
Once you have all the necessary items, the “Trade” button will become available. Pressing the button will complete the collection. In the process, required elements and charges will be consumed and you will receive a new item – the one displayed on the right side of the collection. This item may be required to complete a quest, unlock a location, or simply be an element in another collection.
Additionally, you will receive the rewards listed in the top ribbon of the collection. They may include gold, experience points, and certain useful items.
Some of the collections can be completed more than once, by collecting all of the required items and pressing “Trade” again. This will grant all of the collection’s rewards one more time.