In the OSR it’s well known that there is no reason for monsters to follow the same rules as player characters. Nor do they have to follow the same rules with each other. Each monster should be an opportunity to try new mechanics that keep your players in suspense.

Although most systems rely heavily on their monster manuals, a tradition born in AD&D 1e, games like Lamentations of the Flame Princess and Into the Odd don’t even have an official bestiary, proposing instead that each monster be unique, ad hoc to each adventure.

Following this philosophy we have the monster as trap. In Crypt of the Goddess I designed an encounter that blurs the boundaries between trap, monster, architecture, topographic accident and trick: the Gaslime.

The slime is a traditional fantasy game monster, but its way of attacking is not always like that of other monsters.

I originally designed this encounter for my LotFP campaign, the mechanics are simple: the monster takes the form of a constant fall of semi-gaseous fluid. Players know what they are seeing, or at least suspect it. The monster doesn’t make an attack roll, the attack is automatic when the adventurers cross the liquid (as when crossing a waterfall). The players don’t make a saving throw either, the damage occurs in 100% of cases … unless they come up with a way to avoid contact with the acidic substance.

The trap is presented clearly. Rather than finding and deactivating it, the objective of this meeting is to force players to make a decision. To suffer inevitable damage when accessing the following area? Taking a different direction? Or looking for a way to cross and avoid damage?

The difference between this trap monster and others is not only that it doesn’t make an attack roll (after all, in Into the Odd, which is the system I used in Crypt of the Goddess, there are no attack rolls), but in that it doesn’t allow a saving throw either.

This type of encounter works when the players have at least a small probability of surviving, either because the risk is obvious (as in this example) or because the mechanics allow it (such as saving throws).