Although a
creature's
type and species determine many of its traits and
abilities, GMs are encouraged to alter a
creature's physiology, behavior,
abilities, tactics, and defenses when it serves the story or to confound players who think they know everything about their opponents.
The rules provided allow GMs to build custom monsters and ascribe special qualities to them. When designing a creature, the GM should also think of ways the creature can be defeated. From the heroes' point of view, a creature's
weaknesses are more important than its
abilities. Assigning
weaknesses to
creatures gives under-powered or poorly equipped heroes a fighting chance.
Table: Sources of Weakness lists many sources to which a
creature may be vulnerable. A source can be a specific object, location, substance, sound, sensation, or activity. How the
creature interacts with a source of weakness is left up to the GM, although most sources must be in close proximity to the
creature (if not touching the
creature) to affect it. GMs may roll randomly on the table, choose a source that suits the
creature, or devise their own.