Bythos

When players try to break the universe—and they always do—it’s nice to have an in-game reason to convince them why they shouldn’t…or punish them in a game-appropriate way for going ahead.  Bythos aeons will quickly teach any time-traveling parties why mucking in the fourth dimension is a no-no.  And parties trying to take advantage of a planar rift may have to delay a bythos who wants it closed—if it’s even possible to delay a master of time. (GMs might want to be careful about how they run bythos combat, though; the confusion of a few Temporal Strikes should be fun, but a heavy succession of time-related effects may frustrate players in a bad way, rather than in a constructive one.)

An artifact allows users to go back in time 12 hours.  If they overstay their deadline, they attract the attention of a bythos tribunal.

A prophecy fulfilled returns a king from his watery grave on a boat pulled by swans.  But a bythos determines someone must replace him outside the time stream—likely a cavalier or aristocrat of equal level—and is implacable in its search for candidates.

The plane-hopping corsairs of The Mendicant come screeching into town through a rift in a burst of gatefire, two bythos hot on their tails.

—Pathfinder Bestiary 2 10