This skill represents a direct knowledge of the existence of the Old Ones and their servants, their goals and methods (to whatever extent that the Old Ones can be said to have goals or methods).
Mythos Lore provides a general knowledge of the names and natures of the Great Old Ones, their servants and servant races, and the rituals and traditions of various cults.
While it does not represent a direct knowledge of Mythos-related spells and rituals, such wisdom is often found in conjunction with Mytbos Lore, and Mythos Lore is also the primary skill used in researching new or lost spells and rituals.

　The secrets of Mythos Lore are deeply hidden, and this skill cannot be obtained through conventional research channels.
Generally speaking, PCs should not he allowed to begin the game with Mythos Lore - it should only be obtained through play.
You can only learn Mythos Lore when a source of information (like the Necronomicon) is available.
Direct contact with the Old Ones or with cultists is, of course, an acceptable source of Mythos Lore.
The GM will determine the maximum potential increase in Mytbos Lore possible over the course of a given adventure.
Once this theoretical maximum has been determined, the PCs will, of course, still have to pay earned character points for any actual increase in their Mythos Lore skill.

　The only way to acquire any semblance of Mythos Lore skill without direct contact with the creatures or objects of the Mythos is through a very extensive study of a wide variety of "conventional" occult traditions.
If one devotes years to a study of human mysticism in all its facets, one may begin to get a faint picture of the dark and inhuman cosmic realities which lie behind the traditions. Hence the default to Occultism. Learning Mythos Lore, however, cames with it a terrible price.
Those who study the forbidden secrets of the Old Ones are frequently made mentally unstable - or even driven utterly insane - by their researches.

　In game terms, for each additional point of skill one gains in Mythos Lore, one must make a Mythos Fright Check (seep. 45).
However, unlike other Mythos Fright Checks, the normal Fright Check number is not halved (though the maximum bonus obtainable through Strong Will and related advantages is still +4).
Instead, the Fright Check is rolled at a penalty equal to the skill level of Mythos Lore being purchased.
If the researcher buys up his Mythos Lore skill by +3, from 11 to 14, all at one time, he must make three separate Mythos Fright Checks, with penalties of -12.13 and -14.

　For these purposes, any result on the Fright Check Table (see p. B95) below 13 can be ignored.
These effects are so transitory as to have no relevance in this situation.
Likewise, any transient effects from higher results can also be ignored.
Results which cannot be ignored include any physical damage or permanent physical transmutations (i.e., hair turning white), any period of unconsciousness of more than 24 hours, and, of course, any permanent changes in the character description - new quirks or disadvantages.
These Fright Checks do apply to increases in Mythos Lore acquired as a default from Occultism.

　There are numerous reported cases of world-renowned anthropologists or other respected scholars (individuals with a Occultism skill of 25 or more) becoming severely disturbed or unbalanced due to their growing implicit understanding of Mythos Lore.

　If a Fright Check result calls for a new mental disadvantage of unspecified type, the most common such disadvantage by far is Compulsive Behavior - Obsessed With Gaining More Mythos Lore, thus creating a descending spiral of insanity.

　This will be one of the most common disadvantages acquired by individuals who pick up a mental disadvantage by failing a Mythos Fright Check (see p. 45). The most common forms of such a Compulsive Behavior will be "learn more about the Mythos," "seek to harness the power of the Old Ones," or some similar compulsion likely to lead him into a situation requiring more Mythos Fright Checks.