Mystics are special units (neither Warrior nor Monster) that players can only employ once they've given their clan one of the Mystic Clan Upgrades. Each of these Clan upgrades adds 1 Mystic figure to your clan's reserves, which you can then use to Invade and battle normally. All Mystics have STR 2 and the special ability that allows them to Invade at no Rage cost. Clans are only able to recruit a maximum of 2 Mystics.
Besides adding Mystics to your reserves, each of these Clan Upgrades grants your Mystics a special ability. If you collect several different upgrades, each of your Mystics will posses all the abilities you accumulated, making them incredibly resourceful and strategic additions to your clan!