One time use powerful weapons, fired from a bow. Most Magic Arrows have an area of effect, like a wooshing nova or dark cloud or fiery blast from the point of impact. Making a Hazard save can let you avoid some or all of the negative effects. Use a Hazard save if using traditional ASU triple class system, or use a new 'Bombardment' save if using the “advanced” classes.
Where do Magic Arrows come from?
Archers make them. I prefer my fantasy settings to be less Wizard and magic user focused, so it is the best and most mystic archers themselves who make the arrows. Some probably form naturally or are made by Gods and Spirits.
After all, the only difference between a hunter creating a special boar-hunting arrow out of the tusk-tip of the biggest hog he's killed and another rolling a half-forgotten dream in morning dew and tying it together with cobwebs to create a sleep arrow is the level and magic bonus of the characters.
12 Magic Arrows
 Night-Sky Blanket Arrow
Firing this arrow up into the sky during the day creates a 'seam' the flows horizontally from the arrow, creating a 'blanket' of night over the daylight. The blanket stays up in the air and then slowly fades away over 6 combat rounds. Creatures of the daylight will have penalties for seeing while fighting under the blanket, despite the bright surroundings just a short ways away. Any competent archer can fire the arrow hard and high enough to cover an entire field or city block. Expert archers could stretch it across a whole city or valley, drenching it in unnatural darkness.
 Honey-Hive Arrow
Bright orange arrow with honey-comb head. Conjures a swarm of bees where it lands, dealing 1d4 damage per turn against all thin skinned opponents nearby. If the arrow hits a target directly, the swarm targets them first. If the arrow's damage on hit kills the target, their corpse bleeds 1d4 honey rations.
 Volley Arrow
This arrow splits magically into dozens of other arrows, and flies in a great cloud. Roll 3 attacks per enemy within a cone, and roll normal damage for each attack that hits.
 Shrapnel Arrow
This arrow deals normal damage, then fires forth a cloud of sharp projectiles in the opposite direction of the angle the arrow hit. Deals 2d6 to those directly next to the arrow blast, and 1d6 for those farther away. Add +1d6 to damage if an enclosed space like a room or hallway. Skilled marksmen can angle the arrows to fire their shrapnel around corners.
 Conflagration Arrow
Creates a liquid explosive that fills in space of pure flame. Deals 2d6 fire damage to all within a small area, if not indoors the range is incredibly small.
 Icicle Arrow
Ice arrows that melts after hitting an enemy. Deals +2 ice damage and leaves no evidence. If used during a winter storm add +2 to hit. Chills enemies, dropping their initiative by -1.
 Knockback Arrow
Has a round cube arrow head made of some kind of stone. When it hits enemies, knocks them back ft equal to damage, and can sometimes shatter heavy armor open.
 Pinning Arrow
Solid metallic arrow of light material and wicked sharp point. Penetrates straight through things and pins them to walls, floors, objects, etc. Requires a save to pull it out or break free.
 Vine Arrow
Arrow made of a tangled up thorny vine. Upon hit creates entangling roots that seek and curl up around all nearby living creatures and objects, dealing 1d4 damage each turn people move through it.
 Glowing Arrow
Arrowhead is replaced by a glowing mushroom head or a small jar filled with firebugs. Upon impact, illuminates and 'spotlights' enemies nearby. This negates darkness penalties to ranged attacks and 'highlights' invisible enemies, making them easy to spot.
 Lightning Arrow
Powerful arrow that transforms into a lightning bolt as soon as it is loosed. Travels to the target almost instantly, granting +1 to hit, and deals 1d12 lightning damage.
 Seeker Arrow
On a miss, this arrow curves around to seek the target again, or at the request of the shooter. Marksmen can control a “swarm” of these arrows to suddenly attack one target all at once; number of arrows they can control at once in a swarm is equal to their level.