When activated onto the Lumberjack, you will be able to get Dark Strange Leaves when chopping down trees.
You will need level 100 of their respective skills (Mining, Woodcutting, Farming) to be able to use the potion.
The Dark Potion description says it is brewable from level 70, but when you click it, it says it is craftable from level 75..
| Small Vial: Stardust Potion - Tree Potion - Seed Potion - Smelting Potion - Oil Potion - Bar Potion - Super Stardust Potion - Woodcutting Speed Potion - Combat Cooldown Potion - Farming Speed Potion|
Large Vial: Stargem Potion - Blue Pirate Potion - Mana Potion - Super Oil Potion - Greater Stardust Potion - Stardust Crystal Potion - Super Tree Potion - Super Combat Cooldown Potion - Green Pirate Potion - Sick Cure Potion