One of the things I've seen several GMs do in the past, something I've done myself in the past, and something a friend of mine is doing for their next session is hand over some of the bad guys to one or more players. The idea is the players - who are either not normally part of the game or their characters are not present for the session - run the villains to keep them involved and take some of the load off the GM's back. It's not a bad tactic, but there is one thing to watch out for: player run villains tend to be a lot more dangerous and deadly than GM run villains. Today I want to talk about why, so you have a better idea of knowing if it is the right tactic for you.
Playing To Win vs. Playing For The Story
The biggest reason player run villains are more dangerous is because players tend to like winning. Now the argument is that you can't win in a RPG, but you can win in a fight or encounter. Generally the villain's win condition is killing the PCs, especially in combat. Now put that goal in front of a player and you can see where this is going.
Some of you may balk at this. After all, it's not like you hold back with your NPCs and villains, right? But there's still a difference. Consider everything you have to do as the GM in a combat scenario - or even non-combat scenario. You have to track the rules, the players, the dice, the PCs abilities. You have to track it all and keep it flowing. The player doesn't need to do that.
While you're worrying about the turn order, the player can be prepping their next turn. While you're helping someone else figure out their round, the player can get spells and abilities ready to go. Basically, while you're busy with about everything GMing entails, the player just has to worry about how they're going to handle the moment.
A Little Extra Viciousness
Along with this extra focus players can bring to the villains there is also that bit of extra viciousness. Without the burden of handling the rest of the world the player can focus on their plans and their contingencies more. Even better - for them - the player doesn't have to worry about their idea being fair, or allowing for a fair shake for other players. They're going to have more contingencies in place as well, just because of the extra prep time.
In short, almost every aspect of the prep and planning that you put into your game, a player can put into the villain.
A Big Bad With Bite
When I was younger I had my cousin - a great GM himself and the person who got me into RPGs - run the BBEG for a D&D game I was running. Essentially I ran the game for my players, but I talked to my cousin every week. I told him what the PCs had done that he'd heard about, and he told me what he was having his organizations do. Effectively, we played a high level game over the phone of his BBEG vs. the "good" Kingdoms in the world, and I ran the full on mechanics game with my players.
This made for interesting events because my cousin thought of things I would never think of. He found ways to win even while the players would feel they had done their job to try and calm them down in certain areas. He reacted to threats to his assets in ways I may never have done.
We hit some of the cliche notes (home village burned down and such) but we hit them in different ways and at different times. Ways that actually hurt the PCs more because there was an actual dedicated brain behind the villain.
A Villain With Purpose
Effectively, the player run villain has purpose. They have purpose because another player is controlling them. Their goal and purpose of being isn't to create story for the game or adventures for the other PCs. Their purpose is to accomplish their goals and make their plans come to fruition. It can be a beautiful thing, but it can also be a deadly thing. After all, the bad guy doens't have the same moral restrictions of the good guys, and let's be honest: PCs can get a lot done even when kept in check.
Fair Fights Means Unfair Fights
Think about the tactics your players use on the regular. PCs can be very creative and very vicious. THey lie, cheat, steal, and betray depending on the situation. They'll find ways to kill while putting themselves at no risk. They have an edge in most fights not just because they're the center of the game, but because they frequently use tactics and approaches that just aren't sane for the world - until a PC does it. When you have PCs on both sides though, that balance evens out...and that means none of the fights are going to be fair.
The villain will have hit squads set up to ambush PCs. They'll hit them at places they don't expect. They'll burn cities down to remove the obstacle. Every tactic your players have conceived is open to the villains with a villain PC.
This can mean dead PCs. It can mean hurt PCs. It can mean captured PCs. Most of all, it can mean defeated PCs.
Still, if you're looking for an extra challenge for your game. it's not a bad idea.