Well guys, I'm wondering if it's even possible to do this. Whenever you clear a quest or a mission, the mob dissapears/becomes invisible for the player which did it, but not for the other players.

Is that possible? If yes, how would I go about that?

Raimo H.
The only solution I can think of for this would be to not give the mob an icon, and then use an image object sent to each player's client if they should be able to see the mob. So basically the mob is 'invisible' but certain players can just see an image of the mob. When you want the mob to disappear for a certain player, you just delete the image object for that person.
You can use images. If you're not familiar with them, they're like overlays but you can control which clients can see them. Instead of giving the quest giver an icon, you make them blank and attach an image to them. When you complete the quest you remove the mob's image from the client.images list and the player won't see them anymore.

The catch is that the mob still exists. The player can bump into them and things like for(mob/m in view()) will loop through these hidden quest givers.
In response to Forum_account
Great minds think alike! I beat your reply by one minute though :P
In response to Gunbuddy13
No fair, I was typing on my phone! :-)