Shamans can banish Ghosts, obtain rune fragments, and also lead military marches and help to develop your Town. They can only be summoned when the Palace has reached level 12. 

To summon a Shaman, you need to build the Hall of Mysteries, and also have enough Charms, which are special items. You can get the latter for passing checkpoints in Ghost Hunt Competitions, find them in Jotunheim Monsters' Coffers and Invader's Treasures, or purchase them as part of Bank offers. The Charms of Baggi the Big are also available in the Clan Store. 

Charms are also required to increase a Shaman's Initiation stage and unlock new levels. Every Shaman has a total of six Initiation stages and 60 levels. The higher the stage, the larger the bonus that the Shaman provides and the higher the level that the Shaman can reach. 

You can activate the Shaman's Double boost and temporarily receive a copy of your Shaman that possesses the same stats and grants the same bonuses.

You can find more details about Shamans via this link.