I’m pretty sure I wasn’t alone in that, when I first saw Mythic Adventures, I thought that it was going to be the Pathfinder equivalent of the D&D 3E Epic Level Handbook (which took the standard character classes past level 20). Instead, while Mythic Adventures goes for the sort of high-powered feel of the Epic Level Handbook, Mythic Adventures can be used with characters of any level (note that the concept of ‘epic’ does exist in Pathfinder, in reference to anything above level 20, but to the best of my knowledge there are no formal rules for character classes above level 20).
At its core, Mythic Adventures presents a system that sides alongside the normal character progression. A mythic character has a normal class that is leveled up with normal experience points, and a mythic class (called a path) that is leveled up (they’re called tiers instead of levels though) with mythic trials (which is essentially a milestone system). The mythic path doesn’t have to, but for thematic and synergistic reasons probably will, be a mythic counterpart to the character’s standard class.
There are six mythic paths – archmage (arcane magic), champion (offensive martial combat), guardian (defensive martial combat), heirophant (divine magic), marshal (leadership), and trickster (any of the tricksy, skill-based classes). Martial combat types probably have the most options, as they can easily go into the champion, guardian, or marshal (depending on how the character is built; paladins are particularly suited to the marshal path). Bards fit into either marshal or trickster. Most other classes naturally slot into a particular path (e.g., any class that is primarily a spellcasting class).
All mythic characters gain a standardized bonus at every tier, a path ability (from a lengthy list) every tier, a first-tier ability form their path, and a 10th-tier capstone ability from their path. As they tier up, mythic characters gain a feat every other tier, a skill boost every other tier, automatically stabilize when downed, gain a pool of mythic power (and then a variety of ways to use it, such as a bonus dice, rerolls, or the ability to end negative status effects), an initiative bonus, the ability to fully heal at every long rest, super benefits from saving throws, and eventually virtual immortality. The first-tier special ability is one of three options for expending mythic power.
Path abilities have a multitude of effects (some of which require the use of mythic power). The archmage can auto-succeed at concentration checks, auto-identify spells and magic items, gain energy resistance, or prepare spells without a spellbook. The champion can leap to attack creatures in the air, gain a climb speed, increase in size, or apply feats like Weapon Focus across a whole class of weapons. The guardian can increase armor bonuses, give allies bonuses to saving throws, disarm with ranged attacks, or parry spells. The heirophant can cast touch spells at range, expand the wild shape ability to allies, or spread out the healing from single-target healing spells. The marshal can, well, a whole mess of things that involve boosting allies or have a forceful presence. The trickster can roll twice for skill checks, gain control of constructs, run across walls, or drastically extend potion durations.
The capstone abilities are duly impressive. The archmage gains spell resistance and gets much harder to save against. The champion can re-roll every time they miss on an attack. The guardian takes half damage from attacks. The heirophant gains damage resistance, heal easily, and their spells are also harder to resist. The marshal grants initiative re-rolls to allies, as well as super surprise rounds. The mythic trickster treats everyone as flat-footed.
One commonality with many of the above abilities is not working, or having a lesser effect, against mythic foes (for example, not allowing a saving throw if the target is non-mythic, or removing status effects from non-mythic sources).
Of course, there aren’t just mythic paths. There are mythic feats – these are most commonly souped-up versions of standard feats (mythic Combat Expertise, mythic Fleet, mythic Power Attack, etc.). Similarly, there are mythic spells, which are primarily mythic versions of existing spells (e.g., mythic fireball, which can set things on fire), which are cast by expending mythic power along with casting the normal spell. There are mythic magic items (including artifacts). And there are mythic monsters, which again are of the “mythic version of a monster you recognize” variety.
The contents of Mythic Adventures are rounded out by a GM chapter on running mythic adventures, and, well, a mythic adventure (Fire Over Blackcrag). In the adventure, the PCs are imbued with mythic power in order to face a mighty foe (that’s going to cause a volcano to erupt and destroy the town).
Depending on how you look at it, Mythic Adventures is either very successful, or isn’t successful at all. On the one hand, it presents a fully fleshed out system that (for the most part) works smoothly with the standard mechanics and is flexible enough to incorporate more recent additions such as new character classes (although, of course, doesn’t have things like mythic versions of newer spells, feats, or monsters). It gives a way to have characters be more powerful without just being higher level. I could see it being fun to charge up some characters on a short-term basis to take on some cosmic threat, then power them back down to their normal levels to continue on. Because that power-up isn’t just getting more levels, it isn’t something that the characters will then replicate as they normally progress.
On the other hand, I don’t know that any of that feels particularly mythic, as compared to Pathfinder generally. Sure, the characters can have mythic adventures, ridding the land of evil, saving the country, and so forth … but can’t all Pathfinder characters do that? The abilities of mythic characters are different, but are they any more “mythic?” Is an 8th-level wizard/4th-tier archmage any more mythic than a 10th-level wizard (both of these being CR10 threats). The former can cast mythic spells. The latter can cast higher level spells. Similarly, how much of a difference, from a ‘feeling’ perspective, is a mythic ogre as compared to simply advancing an ogre? Is the mythic version of Cleave much different from the multitude of feats going up the Cleave chain? Is temporarily juicing characters up with mythic levels much different from temporarily giving them really potent magic items (after all, the characters in Fire Over Blackcrag get their mythic levels when they … pick up some items)? There is what I take as a gesture in this direction in that, as mentioned above, many of the mythic powers are ineffective (or much less effective) against other mythic beings – in a game with mythic and non-mythic foes, the PCs may notice a lot more resistance from the mythic ones (and non-mythic PCs, once ascended to mythic status, may find formerly difficult mythic foes much softer). To me, however, this does not do enough to make the mythic rules feel enough more ‘mythic’ than a setting that is already replete with mythic adventures.
So, I think that Mythic Adventures has a place, if you’re looking for something different to juice up your campaign for a little bit. It just doesn’t really feel “mythic.”