Sacred arrow

are arrows imbued with spiritual power, and a tactic most often used by someone with a high-level spiritual powers.

If one is any kind of an heir to spiritual power (i.e. or, Onmyōdō, etc.), even lower-level Reiryoku/spiritual powers, then one can channel that power through a physical medium (i.e. a Shakujō, a Dokko, a Rosary, or an Ofuda), keeping up a constant flow of power for use in close-range attacks against yōkai.

In the case of arrows, however, as a long-distance weapon, a higher-level of Reiryoku/spiritual powers are needed, as the arrow must be able to carry the charge of Reiryoku over a distance to its target, away from the wielder, without losing strength and power. Although they are not always considered a very accurate attack, it is very powerful and deadly coming from the hands of a master archer.

They can also blessed/annointed prior to make them more powerful. Furthermore, exorcism arrows can be versatile, with uses beyond the simple expedient of vanquishing a Yōkai; they can be also be used for other tasks such sealing (Fūin-no-Ya), barriers (Kekkai-no-Ya), etc. They can even be made to be fine-tuned against a specific opponent (as seen when Kikyō used soil from the cave where Onigumo became Naraku on her arrows, making them effective against him (although Naraku became powerful enough to be uneffected, his offshoots were all vulnerable)).

Sacred arrows are the weapons of choice for the miko such as Kikyō, Kaede and Kagome. They are powerful enough against yōkai weapons as to force the yōkai sword, Tessaiga, to transform down from its full-fledged form to its dormant katana state. Kagome has even shown that this attack can be combined with the Tessaiga's attacks, and multiply the amount of damage done.

While it is hinted at that arrow shafts intended for use as exorcism arrows are made from the fallen branches of spiritual trees such as the Goshinboku, however, the supply of which would not necessary meet with the demands; as seen in the times that Kagome was able to use ordinary arrows and channeled charges of her Reiryoku/spiritual power through them to great effect (the source of the arrowheads isn't mentioned either).

Trivia

 * In the manga version, upon Kikyō's resurrection, Inuyasha is able to cut Kikyō's sacred arrow in half, while keeping the Tessaiga in the transformed state . Instead, in the anime version, Tessaiga's transformation is disabled, with Inuyasha being hit by it.