Rats!

Introduction: An encounter intended to ratchet up the tension and deliver an “oh crap” moment at the end. I ran this in a game of Deadlands Reloaded, but I am presenting it as a general concept for use in other games.

Setting/system: This could be adapted to almost any setting, just by changing the specific enemy, though it will probably sit best in a game with some horror elements. System-wise, again it could be adapted to a lot of different systems, but the mechanics would need tweaking depending on the damage system (see below).

Description: These four rooms could be absolutely anything. I ran them as servants’ quarters in a haunted house, but any small set of rooms would work. They should be laid out in such a way that final reveal is in the last room visited – in the layout I’ve presented, if the players head north first, just move the internal door to the east room. Also, there should be some in game reason for them to push through to the final room and not retreat early – most likely, they’re looking for something and need to search every room.

Encounter: As the players enter the first room, they see a couple of small creatures scurrying about (I used rats, but any small creature that is not particularly dangerous on its own would work). As the players progress through the rooms, the creatures appear in larger numbers and become more aggressive. There also appear to be an unlimited number of them; the PCs should be encouraged to keep moving, rather than making a stand. As the players enter the final room, they see a closet door (or setting appropriate portal) bulging and straining. As soon as they get close to it, it bursts open with a flood of creatures – this (combined with the mechanics below) should be an “oh crap” moment for the players. The PCs’ goal may or may not actually be in the closet, but in any case, the mass of enemies should ideally be dealt with through clever thinking, rather than simply fighting it as a normal enemy.

Mechanics: These are the mechanics I homebrewed for this encounter; they are specific to the Savage Worlds system. Further down, I will make some suggestions of how the mechanics might be adapted to other systems.
Each swarm of creatures is represented by a single die – the bigger the die, the larger the swarm. When the swarm attacks a PC, there is no attack roll, it simply does damage by rolling the die (this represents each individual creature having a chance to hit for one damage… the number rolled is the number that hit). The first swarms are only d4 or d6 – in Savage Worlds, this is unlikely to damage the PCs at all (for those unfamiliar with Savage Worlds, damage has to clear a toughness threshold before it has any effect). The swarms progress through the die types as the players progress through the rooms; d8/d10 could potentially hurt the players, d12 is likely to hurt the players, d20 swarms were definitely dangerous. If the players attack a swarm, they do make an attack roll, but don’t bother with damage; melee and ranged attacks reduce the die type of a swarm by one, while area effects reduce the die type by two.
This system worked reasonably well on its own merits, but the primary reason for it was the moment that the closet door burst open, a tidal wave of rats spilled into the room and I chucked a d100 onto the table. Having spent ten minutes engaged with the mechanics above, the simple sight of the d100 was enough to deliver the message “this swarm will kill you”.

Other systems: Adapting this mechanic to other systems is mostly a case of playing around with the dice. For example, in a system with D&D style hit points, you could just use d6s, but increase the number of dice; start with small swarms of 1d6, through medium swarms of 3d6, up to big swarms of 5d6. When the players open the closet, you slap down one of those little box sets of 36d6. In a system with static damage, you could use increasing stacks of poker chips to represent the damage. The goal is to present a very logical progression with a huge jump for the final swarm, so that it is instantly and visually beyond the PCs abilities to fight normally.

Final note: The setup of this encounter means that the final swarm will likely kill a PC if it attacks one of them – it needs to be that deadly to elicit the intended reaction from the players. You may want to consider ahead of time whether that is an acceptable result for your campaign and players. If you want to deliver the scare, but don’t really want any actual deaths, you can always have the d100 swarm spit out a couple of d20 swarms while it organises itself to attack; this will give the players a moment’s breathing room to react or run.