You receive this quest from Elisedd, who is at the entrance of Lydney Park.
She asks she'll give you the key to the park gate if you to bring her 20 pieces of Blackthorn for her to make Blackthorn Tea.
You will receive the Gather Blackthorn quest.
You can find Blackthorn in the Boscawen Marsh.
Go to Boscawen Marsh.
Tap on the Black Bushes and you will receive 4 Blackthorn.
Keep doing that until you have 20 Blackthorn.
Return to Elisedd in Lydney Park.
The Gather Blackthorn quest will be solved.
You will lose 20 Blackthorn.
You will receive Elisedd's Key and a rune gem.