Immune: constructs, kami, ooze, plant, qlippoth, undead (usually), vermin; paladins of 3rd level or higher

A frightened creature flees from the source of its fear as best it can. If unable to flee, it may fight. A frightened creature takes a –2 penalty on all attack rolls, saving throws, skill checks, and ability checks. A frightened creature can use special abilities, including spells, to flee; indeed, the creature must use such means if they are the only way to escape.

Frightened is like shaken, except that the creature must flee if possible. Panicked is a more extreme state of fear.

Marius, Scholai Armsmaster Says: If a shaken character gains the shaken condition again, they become frightened. Even at first level, there are several necromancy spells that can cause the frightened condition.  Most of these spells, such as Cause Fear and Fear, cause a creature to become shaken even if they make their saving throw.  A successful Intimidate check (DC 10 + HD + Wisdom modifier + situational saving throws against fear) will push them over the edge so that they run from battle entirely rather than simply dealing with a few penalties.