The Mental Era is the fifth set with an emphasis on mystery, cunning, and forbidden secrets.  It has the players see the world through the eyes of their everyday worshipers - the commoners of ancient prehistoric societies.  It's not really possible to talk about the Mental Era Set without spoiling it, so I'll cut this off here.

This is a low-scale set based on the Wheel's mental-axis and adjoining colors, so Pfi (Intelligence), Quae (Knowledge), Vei (Wisdom), Jutu (Dexterity), Glei (Perception), and Sace (Charisma).  100 cards of each color are represented (only a few as multi-colored cards), with 300 total cards in the set.  There may be as many as 150 additional special story cards, likely the Greater, Main, and Lesser Gods of these colors.  As a low-scale set, card values range from 1 (common), the lowest in the game, to 1K (mythic rare) and the story roughly covers a period from 200 thousand years ago to 20 thousand years ago (or the period just before conventional history).  While the set is low-scale, it occurs late in the game when most cards of most colors and scales are available.  As a result, gameplay is mostly focused on mental plays of revealing and concealing cards, then escalating or transitioning strategically to a dominating advantage.  Players that master escalating the scale of the encounter or transitioning to new strategies can begin to understand how a lowly mortal might be able to challenge and defeat the all-powerful gods.