NATURE SPIRIT QUEST WALKTHROUGH
After you've talked with Drezel, head through the portal and in to Morytania. Go south-east until you reach a gate with ulizius outside it. Walk through the gate (ignore the warning), and go as far south as you can. You should see an island.
Go to the southern side of the island, and jump the bridge. Try speaking with Filliman Tarlock and he will only speak gibberish.
Amulet of Ghost Speak: If you already have it from the restless ghost quest, jump to the next step. If you don’t have the amulet, here's what you do: Go back to Drezel and tell him about the spirit, he will tell you about the amulet. You then have to go to the Lumbridge swamps and talk with Father Urhney in the most south-eastern house there. He will give you the amulet. Go back to Filliman.
When talking with Filliman try to convince him that he's a ghost, and he won't believe you. Go and pick up the washing bowl on the table to the west of Filliman, and under that you should find a mirror, pick it up and use it on Filliman. He will now believe that he is in fact a ghost.
Filliman will talk about his journal, to find this, search the Grotto, and then speak with him again. Filliman will speak about becoming a Nature Spirit, he will give you a druid spell and tell you that he need three things:
Something with faith: Go to Drezel and ask if he can bless you, he will tell you that you are the "something with faith". After that go back to Fillimans island.
Something of the nature: Cast the druid spell near some of the dead logs. When you've done that, pick a fungus up, and head back to Filliman.
Something of a spirit-to-become freely given: This is your spell.
Then search the 3 stones on the ground around Filliman and use the fungus on the nature stone, the spell paper on the spirit stone, and stand on top of the faith stone yourself. Speak with Filliman and say that you have figured out the puzzle. He will cast a spell, and become a nature spirit. Enter the Grotto and speak with Filliman.
After you've entered the Grotto and spoken with Filliman, he will ask you to make a silver sickle. To do this you need a mould from the Al-Kharid crafting store, and a silver bar. Use the silver bar on a furnace (closest is in Al-Kharid), and make a sickle. Go back to Filliman in the Grotto, and he will bless it for you.
Note: Make sure you have prayer points, you need them to use the sickle.
Now he will give you a druid pouch and ask you to kill three of the ghasts outside. To do this, you will need to right click your sickle and choose "Cast bloom spell" (you can only do this if you're not wielding it) near some logs and dead bushes. Pick up the pieces of nature that appears around you. When you have three, choose "fill" on your druid pouch, and then try to locate some of the ghasts.
When a ghast tries to attack you now, they will become visible, and you can attack them. Kill three and go back to Filliman. This completes the quest.