Slag

Slag is an element introduced in Borderlands 2, and is apparently a by-product of the Eridium refinement process. Slag elemental weapons have a chance to coat enemies in slag. Slagged targets take increased damage from all non-slag sources.

In the time of events of Borderlands: The Pre-Sequel slag, being newly discovered, is mentioned by Jack only on single occasion, and is not accessible for Vault Hunters, but its effect still can be applied to them by some enemies.

Borderlands 2

Effectiveness

Slag damage has no bonuses or penalties against any non-slag elemental target. In Normal Mode and True Vault Hunter Mode, once a target is slagged, any non-slag damage that target suffers is doubled, including effects from elemental affinities, critical hits, melee, and damage over time.

The slag-coating effect lasts 8 seconds on normal enemies. However, slag effects only lasts for 1 second on certain raid bosses. Enemies that are slag elemental resists slag damage, and enemies that can't be affected by damage-over-time also can't be slagged.

In Ultimate Vault Hunter Mode, Slag effects are boosted to last longer(15s) and cause triple damage (+200%) from non-slag sources. Further slag damage against slagged targets is increased by 50%.

Story Involvement

After Handsome Jack began researching the properties of Eridium, he also discovered the mutagenic properties of slag, and commissioned an extensive range of tests in the Wildlife Exploitation Preserve. Information given in the mission Doctor's Orders indicates that while many of the tests were conducted on Pandora's wildlife, Jack also ordered the use of human test subjects as well. These would be herded like cattle into the testing facilities where the scientists were forced to experiment on them under duress. The results were often fatal, but at least in the case of Bloodwing there are indications that some test subjects survived the process, but were left in a horribly mutated state.

Many slag pools can be found throughout the Arid Nexus - Boneyard and Arid Nexus - Badlands, usually below or near the Eridium pipeline.

In The Pre-Sequel, Jack uses slag to power the Eye of Helios by feeding it to the Destroyer's disembodied eye. When Moxxi has Jack's Vault Hunter overload it with slag to engage the failsafe, she actually used it to destroy the Eye after restarting it, thus keeping Jack from using the eye for malicious purposes.

Sources of slag

Though introduced in the mission Rock, Paper, Genocide: Slag Weapons! at around level 8, slag weapons will not spawn below level 15. The only exception is the mission item given for use in the aforementioned mission, as well as slag-only legendary weapons such as the Slagga which are obtained prior to level 15.

List of Unique/Legendary/Seraph weapons and items that always deal Slag damage:
- Chulainn
- Slagga
- Florentine
- Grog Nozzle
- Antagonist
List of Slag elemental skills:

Notes

Purple barrels slag nearby targets when destroyed. They also damage nearby enemies like other barrels, but about half the normal amount, and without a damage-over-time effect.
Slag weapons have a higher chance (1.5x) to apply elemental effects compared to similar weapons of a different element.
Almost all enemies can be slagged, although some of them, particularly larger bosses, will not change in appearance.
- Slag elemental enemies cannot be slagged and are resistant to it.
- Enemies immune to elemental damage over time likewise cannot be slagged. These include crystalisks, constructors, The Leviathan, Jackenstein, Piston's Blimp, Badassasaurus, and Saturn.
- Most raid bosses can be slagged, but the effect only lasts for one second. Bosses include Terramorphous, Hyperius, and Voracidous. Pete the Invincible and Master Gee the Invincible are not affected, and can be slagged as normal.
Slag coating stays on top of any other status effect, allowing for easy identification of slagged targets.
Spiderants, threshers, and sand worms remove slag effect, along with other damage over time, when they burrow.
When Krieg's Release the Beast is active, his extra melee bonus added to a slagged enemy can do damage upwards of 1 million and can kill non-badass enemies in one hit if a critical hit is achieved
In real life, slag is a by-product that is produced when separating metals from ores - it is usually composed of metal oxides and silicon dioxide.

Borderlands: The Pre-Sequel!

As Handsome Jack mentions in the dialogue of Eye to Eye, slag is the substance that allowed him to amplify the power output of the Eye of Helios' enormous laser (actually the eye of Destroyer). Unknown to him, excessive slag usage on the eye could cause space-time instability, and this feature was used by Mad Moxxi to manipulate Jack's Vault Hunters in an attempt to kill all of them and destroy the eye.

Another presence of slag in The Pre-Sequel is more subtle. Its effects (both visuals and damage multiplication) can be applied to Vault Hunters, though rarely, by ranged attacks of various Guardians, Lost Legion Eternal Magi, and The Sentinel.