When killing mobs in the RP HQ, a Majin had spawned as part of the Dark Devil World Boss event but it was invisible.
If you look at the chat log, you can see it giving me the 'Take my whip' debuff but I could not target it or hit it with an AOE skill. Even at the start of the gif, you can see that I received it's debuff.
- It is probably worth noting that the mobs spawn without me even accepting the Dark Devil quest, as there is no icon in the bottom-right.
Here is a gif: