Don't tell me how..i want to find out my self but if there isn't a way then i wont bother to continue to try and think of a way to do it.

Is there a way you can make a mob like an NPC appear invisible to 1 set of players who have finished a quest and 1 set who hasn't can see it?

Yes or no
Thanks in advance..
In response to DarkCampainger
How? :)
In response to Komuroto
If your quest system is linear, you could set the invisibilility of the NPC to a certain value, I assume that it's the beginning NPC so I will make it 100. Also start off with the players' see_invisible var at 100.

Once the player completes the quest, you make their see_invisible equal to 99, so they cannot see the NPC. Be aware that they'd still be able to interact with the NPC in a few ways, like if it was dense they would still collide with the NPC. And I do believe that that NPC would show up in range() procs' return values as well.
In response to Oasiscircle
hmm then what about spawning an NPC, once a quest has started, and then deleting them?

So the quest giver would be a tangible object like a...poster board?

and the npc in question would only spawn during that quest?

How would i go about that instead?