Darklighter Arrows

Darklighter Arrows are arrows dipped in an unknown poison forged by Darklighters to fire at their targets with a Darklighter Crossbow.

These poisonous arrows are fatal to Whitelighters and Whitelighter-Witches unless they are healed in time. The Elders are also vulnerable to the poison in the arrows^[1], and remain in the Heavens to protect themselves from this threat. Other magical beings, such as witches, demons and Darklighters themselves are not affected by the poison, but may still be injured or vanquished by the physical impact of the arrow itself. However, powerful beings may resist the attack.

The poison in the arrows does not kill Whitelighters instantly, but kills in a slow and painful manner. Those infected by the poison are severely weakened and their magic is neutralized.^[2]

Notes

In "Love Hurts", Leo warned the sisters not to touch the arrow as it was poisonous. However, the poison does not affect witches or mortals. This could either be a mistake in writing or simply a mistake by Leo due to being badly poisoned and confused.
The last evil being ever vanquished on screen was a Darklighter, who was vanquished by Paige by orbing his own arrow back at him in "Forever Charmed".
While the Charmed series is consistent in its use of the term "arrow" in regards to Darklighter Crossbows, projectiles fired from a crossbow are technically more accurately designated as "bolts" or "quarrels".
It's shown in "Blinded by the Whitelighter" that Darklighter arrows can be used by Warlocks to harness the powers of Whitelighters, much like they would use athames on Witches.