A bonfire is lit immediately south-west of Rellekka in the Fremennik Province during the 2012 Winter event by Vitharr and players to lure Sköll into the plane of Gielinor, which mistakes the bonfire as the sun, to kill it.
120 logs of any kind is required to make it burn with the maximum brightness, then Sköll would spawn, and the bonfire would go out. It is impossible to add logs to the bonfire while Sköll is alive.
As the type of logs makes no difference, a cheap and fast way is to take willow logs from the Lunar Isle bank, get any member of the Moon Clan to teleport you to Rellekka for not having a Seal of Passage, deposit logs to the bonfire, Lunar Home Teleport back to the Lunar Isle, and repeat. This requires completion of Dream Mentor, unless you are willing to get a new Seal of passage from Brundt the Chieftain every time you pass by the long hall, but this would still require Lunar Diplomacy to be at least started. With the help of a Spirit terrorbird, you could summon Sköll in three short trips, costing less than 2000 coins.