
They happen every, what, 3 days? One boss monster is chosen to attack your kingdom. That monster replaces the regular on-land merchant, and you have 48h to get rid of it. If you happen to kill it, the merchant will appear, but it seems (need confirmation), that one day would be reserved to the monster, and another day to the merchant, which could explain the fact that killing the monster on day 1 would make the day-2 merchant appear, and when the second day started, there would be no merchants, because the merchant had already appeared.

The monster has 3 levels: easy, hard and brutal (each of them, the creature will show its appearance as they usually do for 4, 5 and 6 stars); and you must spend a great amount of energy to engage in each battle: 30, 50, 70 (need confirmation). The differences of difficulty between each level is greatly substantial. All rewards are very good for their level of challenge.

The first one was the Blue Stegosaurus, which happened to absorb and be healed by Yellow attacks.

Now, the second one, is the Red Asura. It is not immune or heal with Blue attacks. However, it has some nasty debuffs, as high as 95-98% of effect upon weakening defenses or diminishing drastically your healing output. I'm having a very difficult time with the Hard level, still mulling over a way to take that fiend down. Of course, it's about dispelling these awful ailments asap, so I'm thinking on my next team to attack it, because costing 70 energy every time I try it, it's not very productive losing too much.
So, have you guys beat the Asura yet?