The Pathfinder D20 SRD on Mythic Adventures is found at this location
Some important points from the rules:
The Mythic rules present a system for increasing characters’ intrinsic power, making them more like the heroes of ancient myths and epics. Mythic Adventures includes the rules for mythic paths and monsters with the mythic subtype.
Base Mythic character rules are found here.
These are the base Tier 1 Mythic abilities:
Hard to Kill (Ex)
Whenever you’re below 0 hit points, you automatically stabilize without needing to attempt a Constitution check. If you have an ability that allows you to act while below 0 hit points, you still lose hit points for taking actions, as specified by that ability. Bleed damage still causes you to lose hit points when below 0 hit points. In addition, you don’t die until your total number of negative hit points is equal to or greater than double your Constitution score.
Mythic Power (Su)
Mythic characters can draw upon a wellspring of power to accomplish amazing deeds and cheat fate. This power is used by a number of different abilities. Each day, you can expend an amount of mythic power equal to 3 plus double your mythic tier (5/day at 1st tier, 7/day at 2nd, etc.). This amount is your maximum amount of mythic power. If an ability allows you to regain uses of your mythic power, you can never have more than this amount.
You can call upon your mythic power to overcome difficult challenges. You can expend one use of mythic power to increase any d20 roll you just made by rolling 1d6 and adding it to the result. Using this ability is an immediate action taken after the result of the original roll is revealed. This can change the outcome of the roll. The bonus die gained by using this ability increases to 1d8 at 4th tier, 1d10 at 7th tier, and 1d12 at 10th tier.
When a character becomes Mythic, they choose one of six Paths, which give them access to a different set of abilities: