I really like chatdemon's analysis of the overuse of mythic creatures, over at Stocking the Dungeon (http://stockingthedungeon.blogspot.com/2010/06/setting-campaign-mood-mundane.html).
His point: keep the magic alive by NOT using too many mythical creatures. His example was a throw-away minotaur encounter in Keep on the Borderlands. Mine would be sub-level of the Giant's keep, where 20-odd trolls populate a cave room, right next to a cave room with a ancient red dragon, next to a cave room with 10 fire giants, etc.... I found the The Hidden Shrine of Tamoachan to give off a similar feel, too overstocked, if you know what I mean.
I like the idea of an exploration, a discovery, wrapped in a puzzle and a mystery, surrounded by meaningful role-play, with a feeling of building suspense and mounting danger. If you want an exercise in victory via dice tossing, you would play Risk, right?
I am keeping the thrill in mythic monsters, by, number one, making the PCs human-centric. In Arizona Adventures, the different races are totally foreign. There are no random dwarves or elves cruising through town, and a visit to a elven forest or dwarven cave will be weird, and pregnant with danger. Heck, in my campaign, the different human races are mistrustful and competitive, and there is no such thing as a Common language.
The standard monsters in mythic Arizona are giants and dragons. Giant insects, birds, and mammals are also a regular feature of the dangerous wilderness, but they are large versions of regular animals, not mythic at all. The standard crew of humanoid tribes can add to the sense of drama and intrigue, as SENTIENT monsters create many adventuring hooks. But, come on, how deep in the monster manual do you need to dig to create a good adventure?
You should probably be able to number your list of standard/mundane monsters on two hands (e.g. dragon, giant, troll, goblin, orc, gnoll, centaur, werewolf, griffon, etc.). We consider them "mundane" monsters, because they are so common in our cultural myths, your players know of them and expect them to be there.
In other words, mythic creatures should be rarely used, and only used when set up with a meaningful context. In other words, chimeras/hydras/sphinx/medusa are not just part of the regular ecosystem. They a break in the regular fabric of the world, perhaps the creation of some insane mage, perhaps brought to our world from some foreign plane of existence, or working on behalf of some unknown god. Keep them special by keeping them rare, and using them only in the service of a larger story arc.
Of course, in the "mythic megadungeon" all bets are off, anything goes, and the appearance of the wonderous mythic is to be expected. (H.T. http://www.philotomy.com/#dungeon). Let's put it this way: there are dungeons built by human hands for human purposes, and there are dungeons built by dwarves and goblins, but then there are dungeons which seem to have a life and purpose of their own... With no known bottom, with seemingly random and shifting connections to various planes of existence... Every campaign should have at least one mythic megadungeon.
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