Ghosts and Supreme Ghosts
Ghosts are mystical opponents that periodically appear on the Global Map. You can banish Ghosts with the help of a Shaman and receive valuable items, resources, and, most importantly, rune fragments as a reward.
A Ghost's health is determined by its level. Level 1 Ghosts are the most vulnerable, level 6 Ghosts are the least vulnerable. The lower a Ghost's level is, the easier it is to destroy it. All Ghosts are marked in purple on the Global Map.
Supreme Ghosts are a more dangerous type of Ghost. They have substantially more health than level 6 Ghosts. It is more difficult to defeat them, but the reward for doing so will be bigger.
In the "Invaders, Ghosts, and bonuses" tab in the Events window, you can find out which Ghosts are currently on the Global Map.
On the forum, you can check out all the Ghosts that are in the game, and which runes they drop.