Filliman Tarlock is the central character of the Nature Spirit quest. A ghostspeak amulet is required to speak to him. During the quest, the player helps him transform from a ghost into a nature spirit.
He lives in his grotto in Mort Myre Swamp, where an altar of Guthix can be found after completing the quest. This altar gives any player a bonus of +2 in their Prayer stats (e.g. 47/45 Prayer points).
The most efficient way to get to him is to hug the western coast of the swamp whilst travelling south, running all the way.
Filliman can bless silver sickles for the player after they have completed Nature Spirit, though the drop trick is necessary to obtain more than one.
Filliman is later needed during the Fairy Tale I - Growing Pains quest to enchant regular secateurs into magic secateurs in order to defeat Tanglefoot. Random items are required to do so; this varies with each player. The full list can be seen here.