The strong defence potion is a one dose potion only available in the Dungeoneering skill. Using the potion boosts a player's Defence level by 3.0 + 15% of their Defence level.
It is made by adding Buckthorn and Void dust to a Vial of water. This requires level 75 Herblore and grants 170.5 experience in Herblore. This potion can only be made in dungeons with complexity level ≥5.
There used to be a glitch when making this potion. Instead of making the potion, a message used to come up saying players needed 85 Herblore instead of 75. This has since been fixed.