Ring the death knell. Headshots briefly turn this scoped pistol into a rapid-fire, Critical Damage fiend that uses no ammo.
The Knell is Harrow's signature pistol that can use its scope to more accurately land shots at long range. Although its magazine can only hold 1 round, headshots grant it infinite ammunition for a short duration, as well as increased critical damage.
Upon headshots, similar to how the Dual Toxocyst gets a buff, the pistol gets the Death Knell buff. This allows the pistol to consume no ammo while firing for 3 seconds, bypassing the need to reload. Scoring consecutive headshots increases the critical multiplier from 1.5x, 2x, to at most, 2.5x. This buff is refreshed by landing successive headshots.
"Upon headshots" specifically means headshots, and not other weakspots, such as the backpacks of most MOA variants. Similarly, shooting weakspots generated by either Sonar or Detect Vulnerability in places other than heads will have no effect either.
Death Knell's critical damage buff is a flat 1.5x bonus added after critical damage mods such as Target Cracker. For instance, with Target Cracker and a full Death Knell buff your true damage multi (not including headshot damage) would be (1.5x × 1.6) + 1.5x = 3.9x.
As an indication that this buff is active, the scope design while zoomed in will evolve to a different design, as well as a "Death Knell" buff in the buff UI.
The Death Knell buff can be activated by headshotting corpses.
Harrow's Condemn, in particular, synergizes extremely well with Knell as chained enemies leave their heads very vulnerable to being shot at.
Scourge also works particularly well with Knell, as the Bullet Attractor-like fields caused by throwing the speargun are created over the enemies' heads, and all bullets fired on them are guaranteed to land as headshots no matter the part of such enemy's body is the weapon fired upon.
Pressurized Magazine can be very beneficial on this weapon, as it will increase the fire rate during the infinite ammo buff considerably if you can score a headshot quickly after reloading.
Seeker is also a viable mod for this pistol, especially when expecting enemies to spawn in a corridor such as the main hallway in a Void Defense mission, or when trying to score a headshot against an enemy hiding behind cover or Shield Lancers.