Kill Treants to get the 8 Foul Seeds, then use them to combine the seeds into a Seed Bundle.
On the eastern beach you will find a Forest Spirit. He will speak to you, asking you to step into his circle of light and use the Seed Bundle to purify it.
After, he asks you to take the bundle to Orlan's Unsettled Mound and plant the seeds there.
Defeat the Defiled Protector to free Orlan.