Furious battles, perilous environments, and ribald exchanges with NPCs are the meat and potatoes of most games. As GMs, we spend each week asking "how can I spice this up?" This is usually where the specter of 'challenge' appears, enticing the seeker-GM with a spike of difficulty to keep players on their toes or stir the pot of the story. If you're like me, this challenge takes the form of some kind of battle all-too-often.
Don't misunderstand me! Interesting, mobile, novel, or particularly deadly fights are awesome in any game. They should be employed often, with all kinds of moving parts to challenge player tactics and team cohesion. When the time comes that inventive combat just doesn't feel like 'the next step,' that's when you need to employ a MacGuffin.
This term was originated by Scottish writer Angus MacPhail (yes, it's a great pun). In his short story, a character refers to a hidden object in a suitcase which catches tigers in the highlands... a MacGuffin. It refers to a person or thing outside the thrust of story that is, in itself, hidden, irrelevant, obtuse, or arbitrary. You will employ this brilliant story device to create an encounter that will rattle your players. Here are two versions I use when the time feels right:
MacGuffin 1: Enemies have kidnapped an innocent, taken over a fortress, or encamped in a once-sacred space. They must be destroyed! Upon engaging them, though, they are utterly impossible to harm. At the edge of the encounter, or just-so obscured, these enemies possess a gem, crystal, or relic that makes them invulnerable. Until it is destroyed, the heroes have zero chance of victory.
MacGuffin 2: Our heroes must accomplish a task in a decaying environment such as a tidal surge, volcanic eruption, crumbling castle or burning forest. The task itself is quite simple, but the location is utterly deadly. The situation is about to become unsurvivable, exacerbated by chanting wizards, an overloading arcane item, or wondrous machine. The enemies in this scene are a distraction. If heroes do not rally to stop the root cause of the cataclysm, they will be subsumed.
My most recent MacGuffin pitted heroes against an imminent tidal surge in a crumbling town
When executing an encounter like this, be sure your players are aware that death is a very real possibility in the moments ahead. Take a cue from Hitchcock here, openly revealing that someone is bound to die soon. This isn't meta-play, it's a dramatic device to heighten tension, but cloaked as a public service warning.
Our warrior, Gendri, makes his way through the slippery, crumbling ruins to what would be his death
The careful balance of delivering these kinds of scenes is not combat stats, terrain details, or even timing. It is the revelation of truth from obscurity. Players will be seeking why nothing is working, or why they are so imminently doomed. This discovery effort can't be too hard, too hidden, or too abstract to grasp... nor should it be obvious or easy. Yep, it's a narrow tightrope to walk, but you're a game master. You've got this.