Creatures with the fiendish template live in the Lower Planes, such as the Abyss and Hell, but can be summoned using spells such as summon monster and planar ally. A fiendish creature's CR increases by +1 only if the base creature has 5 or more HD.
A fiendish creature's quick and rebuild rules are the same.
Senses gains darkvision 60 ft.
Defensive Abilities gains damage reduction and energy resistance as noted on the table.
SR gains spell resistance equal to new CR +5.
Once per day, the fiendish creature may smite a good-aligned creature. As a swift action, the creature chooses one target within sight to smite. If this target is good, the creature adds its Charisma bonus (if any) to attack rolls and gains a damage bonus equal to its HD against that foe. This effect persists until the target is dead or the creature rests.