Rare Wanted Mission chests are rare and appear randomly, as long as you continue completing your regular Wanted Mission chests.
There are five (5) types of Rare Wanted Missions, one for each corresponding Element. Each one you complete offers the chance for improved loot and ascension items. You complete these uncommon missions the same way you complete your Wanted Missions.
We understand it can be frustrating not knowing when the next one will appear, but be patient...they are worth the wait!