Shadow Tara is only accessible through the stonehenge in Tara. It is the main staging point in Generation 10, Generation 11, Generation 12, as well as many mainstream quests. Shadow Tara was first introduced in the Generation 9 update. See Tara Shadow Missions for a list of shadow missions in Tara.
Within the Tara Shadow Missions are orbs. When hit, the orbs will spawn a wave, or multiple waves, that will need to be dispatched in order to progress in the mission. The difficulty of these enemies are dependant on the difficulty you chose when getting the mission from the Shadow Mission Board.
See for more information on the monsters. The monsters you will see more often in Tara missions are the following:
For more information, visit the Shadow Mission wiki page.