Terraria (Wiki): Dark Harvest

The Dark Harvest is a whip that inflicts an aura of dark energy on the enemy. It is the third most damaging whip and it emits a purple-colored light when fired or swung.

The dark aura flares out and deals damage to both the target and any nearby enemies each time a minion or sentry attacks.

Due to its buff, switching to another whip while the buff is active will improve its speed for a limited time. They can focus on mini-bosses while the dark energy eliminates nearby weaker enemies.

Like other whips, it marks enemies that minions and sentries will attack. If it hits many targets in one attack, the damage dealt by each in-a-row hit will be reduced (10%).

The Legendary modifier is best for the Dark Harvest, providing multiple increases in stats.