The Roots of Evil
Level 200
There seem to be Sadidas in Corruption's dimension. Maybe they can help you get rid of the Horseman, but you'd have to find out what's become of them. The environment isn't exactly welcoming.
Find some trace of the Sadidas in the dimension
Talk to Akronite Guard
Take 1 Luminescent Mushroom to Akronite Guard
Talk to Chief Orobranch
Find the map: Statue of Sadida
Talk to Monotropa Priestess
Take 15 Putrid Sap to Monotropa Priestess
Place your offerings in front of the statue of Sadida
Question the priestess about their religion
Talk to Chief Orobranch again
Find the map: Hermit's Den
Talk to Dogabah Hermit
Take 5 Tentaclobber Tentacle to Dogabah Hermit
Try and enter the Tree of Death
Talk to Dogabah Hermit again
Take 3 Demongrel Seed to Dogabah Hermit
Take 1 Corrupted Water to Dogabah Hermit
Take 10 Corrupted Sprouts to Dogabah Hermit
Go back to the Tree of Death turned into a Demongrel
Talk to Rafflesia the Foul
Examine the heart of the Tree of Death
Talk to Ratathrosk
Remind Ratathrosk of your existence
Head to the Vagabonds' Tree with Ratathrosk's help
Talk to Ratathrosk about the corruption of the location