Hey Creus, sorry for just ignoring your question. For two years...
I was going to create a intro for the game and tell you afterwards. Life happened and I ended stopping the work to focus on other things, and ended forgetting to answer you (sorry). I started working on it again these days and decided to that I will make this game a bigger game, and to not simply put a big intro for a 20 seconds game (It has about 2 minutes). So for now you can see what would be the intro for the game here.
Responding in a more direct way, the ghosts are servants of Nero, the guy that created the potion. Nero was a powerful necromancer and had some hidden temples filled with those servants.
Also, I recently created Sleep Little Dragon, a game related to this one.
Well, that's it. Hope you're well :)