The Scourge is  Harrow's signature speargun, firing fully automatic  Corrosive projectiles that explode and damage nearby enemies in a small radius. Its Alternate Fire allows the very weapon itself to be thrown, inflicting a Bullet Attractor field on the heads of all enemies near the impact point.

Update 33.5 (2023-06-21)

Fixed Furis’ Incarnon Form causing self-damage when shooting an enemy that is affected by Scourge’s Alt-Fire.
Update 32.3 (2023-02-15)

Fixed Scourge projectiles swirling around Grendel after casting Feast on enemies affected by its alt fire.
Update 32.0 (2022-09-07)

Ammo Changes
We understand the power fantasy of these weapons is very popular for some, so our first approach was to keep them powerful, but limit how often they can be used by making changes to ammo pick-up.
Ammo Pickup Overrides
The following weapons now have ammo pickup overrides, meaning they gain less from picking up ammo. This mostly applies to AoE weapons, in hopes that players will become more deliberate with their aim to make the most of their Ammo pools. The following lists the specific ammo pick up overrides per weapon category.
Primary Weapon Ammo Pick Up Overrides
60 Ammo per Pick Up: 

Scourge
Headshot Damage Changes
During playtesting, the ammo changes alone weren’t enough to significantly change things for AoE weapons. So instead of tightening these values to be overly restrictive, we chose to address other factors that make these weapons so strong, such as headshot damage. 
Our approach here is two-fold, to reduce the impact of headshots for AOE weapons, but to also make precision headshots pack a larger punch overall.

Radial damage no longer gains extra headshot damage or triggers headshot conditions.
This is primarily targeted at the Ignis Wraith - as a “flamethrower” weapon, it’s hard to imagine a blanket of elements being precise enough to target weak points specifically. Explosive projectiles themselves can still land headshots, so there is still a reason to aim with most weapons for maximum damage.
SPEAR GUN CHANGES
We have changed the way that Spear Guns function in-game to help increase their viability in mission. Based on overwhelming community feedback we’ve:

Removed the need to manually retrieve your Spear Gun once it’s been thrown. Your Spear Gun now re-appears in your hand using reload.
Removed the need to charge your Spear Gun to throw it, simply Tap alt-fire to throw!
Within the arsenal, ‘Charged Throw’ stats have simply become ‘Throw’ stats.
Spear Guns have also received new throw animations and updated sound FX on reload, plus a new Spear Gun category has been added when searching for weapons in the Arsenal.
Update 31.1 (2022-02-09)

Fixed Javlok projectiles flying side-on to the direction of travel when the Renuntio Speargun Skin is equipped. Also fixes the same scenario for the Scourge/Scourge Prime with the Carcinus Speargun Skin equipped.
Hotfix 29.6.4 (2021-01-06)

Fixed a Dedicated Server script error in Conclave when using the Scourge.
Update 27.4 (2020-05-01)

Fixed the Scourge projectile trail FX appearing distorted.
Hotfix 27.2.2 (2020-03-06)
Reduced the following AoE weapons Radial Damage Falloff from central impact that were all previously 90%:

Scourge: 30% / Alt Fire 60%
Update 27.2 (2020-03-05)

Self Damage Changes

We are getting rid of Self Damage and replacing it with something else: instead of Self Damage, it’s now ‘Stagger’. This change completely removes the chance of killing yourself, and instead now creates scenarios where you will interrupt yourself - or ‘Stagger’ - to varying degrees if you aren’t careful. 
The degrees of Self-Interrupt start with a small stumble all the way to full knockdown depending on how close you are to the center of explosion. Any Mods referring to Self Damage will be converted to acknowledge Stagger.
With this Self-Interrupt system, we have added dozens of new recovery animations that harness a ninja-like recovery experience. By pressing ‘Jump’ at the correct time, you can execute a ninja-skill-based knockdown recovery to ALL in-game knockdowns. This ninja recovery window is indicated by a glowing FX on your Warframe.
In the original Dev Workshop, we said:
As a result of this overall systemic change, Weapons with Stagger will be getting approximately a 20% buff in Damage, with any weapons with AOE receiving a 50% Radial Damage Falloff from central impact.
This is no longer accurate after continued testing. What we are doing now is:
No damage buffs have been added, but any weapons with AOE are receiving ~20% increase in Radius. Additionally, AOE weapons are receiving a 90% Radial Damage Falloff from central impact. This means on the very outer section of the explosion Radius 10% of the Damage will be dealt. Tactics will be deadly - aim true, Tenno. 
Why: Several players brought up the history of the Tonkor and we want to make sure we ship this change in a place that’s conservative in its starting point from a balance perspective. The complete removal of Self Damage does change the pace of destruction with some of the game’s most powerful weapons, so we want to make sure we can iterate upwardly instead of releasing a bonanza of explosions with no other choices.
Update 25.0 (2019-05-22)

33% of clip reloaded every second when Spear is deployed. This allows for a more fluid use of all the Gun Spear mechanics.
Increased Ammo pool from 100 to 200.
Increased Alt Fire's bullet attractor radius from 1.8m to 2m.
Increased Magazine size from 20 to 40.
Increased Primary fire area-of-effect damage from 30 to 55.
Primary Fire recoil reduced.
Hotfix 24.5.2 (2019-03-18)

Fixed missing Scourge hit sounds.
Hotfix 23.1.2 (2018-07-24)

Fixed an issue with the Scourge's throw sounds playing when Modding.
Hotfix 22.12.3 (2018-02-12)

Fixed the Scourge appearing to be rotated incorrectly when viewed on a surface
Update 22.12 (2018-02-09)

Damage from direct hits increased from 30 to 70 (Primary Fire).
Update 22.0 (2017-10-12)

Fixed Scourge not being affected by Adhesive Blast and Firestorm Mods.
Update 21.4 (2017-08-10)

Removed Corrosive radial AoE from Scourge in Conclave.
Scourge's bullet attractor now lasts 3 seconds in Conclave.
Hotfix 21.3.1 (2017-08-04)

Adhesive Blast is no longer equippable on the Scourge.
Lessened the spread from the Heavy Caliber mod when used with the Scourge.
Hotfix 21.0.9 (2017-07-17)

Fixed the Scourge not being affected by Heavy Caliber's accuracy reduction.
Fixed the Scourge damaging or killing players when picking it up while paused in Limbo’s Rift before it hits the ground.
Fixed the Scourge and Cycron Status lasting longer for the Host.
(undocumented) Fixed projectile being able to bypass Nullifier bubbles.
Update 21.0 (2017-06-29)

Introduced.