Impossible Arithmetic

School divination; Level cleric 3, medium 2, psychic 3, sorcerer/wizard 3


Casting Time 10 minutes
Components V, S, M (an offering to the Great Old Ones; see text)


Range personal
Target you
Duration instantaneous


You gain a glimpse of a terrible truth, receiving one truthful answer regarding the Mythos. The material component is an offering to the Great Old Ones. It is up to the GM to decide if the offering is acceptable— rarer or more obscure information requires more exotic offerings. For example, a silver holy symbol of a Great Old One would be appropriate to learn a vague clue about the components required to cast a minor ritual or to learn one fact about a minor Mythos monster.

A scroll of a rare spell of 4th level would be suitable to learn about the goals, methods, or origins of a CR 8 or lower creature. A 1st-level wand or alchemical curiosities worth at least 1,500 gp would be suitable to understand how to perform some rituals without having detailed instructions. An exotic magic item or occult art object worth 3,000 gp would be a suitable offering to learn the likely weaknesses of a CR 15 monster or deduce what Great Old One a secretive cult is likely trying to summon based on a few clues.

Section 15: Copyright Notice

Sandy Petersen’s Cthulhu Mythos, © 2017, Petersen Games; Authors: Sandy Petersen, Arthur Petersen, Ian Starcher.

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