While playing a game with a previous group, in the middle of a combat, I had an enemy caster hidden from general combat, but still casting different spells like heal and other non-damage spells to support the enemies. My PCs never thought to look for him and were becoming frustrated when I would seemingly randomly add health or other effects to the ones they were engaged with.
To make matters worse, I rolled a dice to randomly pick a party member and just told that character to make a will saving throw without telling him why. When he failed, I handed him a note that told him the enemy I was using had just crazed him, and on his next turn before attacking he had to roll a d6: 1-4 was party member he had to attack, 5 was he attack an enemy, and 6 was damage self in some fashion. He rolled a 2 and attacked the corresponding PC. He ended up dealing a critical and killing that PC. At that point anarchy broke out at my table with ensuing fights and eventually two people getting up and just leaving.
The caster wasn't impossible to see; the PCs were just more concerned with beating things up and I tried to indicate he was there. At one point, I did everything but put a giant flashing sign saying "there's someone in that thicket". Three different times, the guy's raven familiar flew from that location, touched an enemy, and returned to thicket. At the end, while explaining to the players, I asked them if they never thought that was suspicious. They all told me they thought it was just a bird and wanted to know how they were supposed to know it wasn't just a regular raven. There was even a sorcerer in the party who had a familiar, who never said anything during the encounter; he just kept spam-casting.
I didn't even intend on this situation to be a combat encounter. The PCs were supposed to be looking for the enemy that was in hiding to obtain some info. Instead, they got to a camp and never stopped to figure out if the people were hostile or not: they just got murder happy. They all knew the spellcaster 1) should have been there 2) was a high powered individual 3) was a recluse who was opposed to conflict.
My question is: was I wrong as DM to force the player to attack other characters? Is that within my power?
Best Answer
Yes, that was fine.
You're the DM: you're there to challenge them and put them in danger, not to keep them all safe.
A lot of this comes down to the kind of game you all play. Statements like "dice rolls should never kill a character" or "you should never have hidden enemies" are total BS. Those are valid agreed-upon social contract items for your group, but they are not generally true statements. Players in my group would recoil in horror at the thought of having either of those rules in place.
Now, some of the problem may be a lack of common understanding of the kind of game you are all in, and it's worth a discussion about what you consider to be the parameters of your campaign - how can characters die, what kind of tactics are fair, etc.
Hidden Casters
This is fine. PCs love to put on Invisibility and cast spells, bad guys can too. I just ran a game two weeks ago where there were melee opponents and then an invisible summoner who was doing a lot of stuff. The PCs figured that spell effects don't just come from nowhere, made Perception checks, cast Invisibility Purge, bing bang boom. Of course, they're smart players... With the familiar, you telegraphed this pretty hard.
Mind Control
OK, players never like mind control, but there's 100 spells that do it and fear you or confuse you or charm you or dominate you. It's a part of the game. Also a valid tactic. Some people claim that this caused the other PC to die with "nothing they could do about it." That's patently false - they have as much as they can do about it as when they get attacked by any other threat (have a higher AC?). There's not always a specific "roll to avoid dying" in D&D. If this was a summoned creature or invisible creature or teleporting-in creature or any number of other things, it would be the exact same lethality.
Maturity
The real problem here is likely one of more of a combination of common understanding of what can happen in the game and maturity. I suspect you're newer gamers and perhaps young. I suspect some of these folks would "flip out" in the same way if they lost a big fight in a MMO or a basketball game, right? That's just general social maturity and what do do about that is out of scope of RPG.SE.
But there's also the issue of newer gamers not knowing all what could happen. If it's the first time they see an invisible caster, then it's quite a shock. Once you've been gaming 20 years that's instead like about the first thing you check for if anything unexplained happens. We have various "if person X gets mind controlled" countermeasures planned out. But for new gamers, the combination of the unexpected with general emotional trouble dealing with loss is a hard combination.
Once you get folks back, it may be worth using this as a teaching opportunity. "Yes, that was hardcore, right? You can do the same thing, be trickier than your opponents! Now roll a new character even more bad ass than the last one!" "Old school" gaming was a continual exercise in this exact thing - requiring you to actually think about what you're doing and not just run forward and grind - and people loved/love it. If they want to play a different kind of game ("I don't ever want to die! I want save points! And nothing should ever hinder my character, mentally or physically!") then you can negotiate the kind of game you want to run and they all want to play.