Effective Against
LeviathanSodium borate, commonly known as borax, is a household cleaning product that happens to be the only known substance that can effectively harm Leviathans. Given that Leviathans are virtually indestructible by any other means — immune to angel blades, the Colt, fire, silver, and all other standard weapons — the discovery of the borax weakness was a critical breakthrough in Season 7.
Usage
Borax burns Leviathan flesh on contact, producing a chemical-burn reaction that causes intense pain and temporary incapacitation. It can be dissolved in water and thrown, applied directly as a powder, or used in improvised delivery systems. The effect is temporary — Leviathans heal from borax burns — but it provides a crucial window for other actions, particularly decapitation.
The standard anti-Leviathan tactic developed by the Winchesters was: hit with borax (to stun), immediately behead (to incapacitate), then separate the head from the body by significant distance (to prevent reattachment). This procedure did not kill the Leviathan but kept it out of commission.
Discovery
Bobby Singer and the Winchesters discovered the borax weakness through systematic testing, trying every substance in Bobby's extensive collection against captured Leviathan material. The fact that a common household cleaner turned out to be the weakness of the most ancient and powerful creatures in creation was played for dark humor in the show.