Speak to Hotpheex in the druidic circle in Taverley. He will be attempting to perform the annual summer ritual to Guthix as a thanks for creating Gielinor, as well as keeping it in balance, as Guthix requested. However, the druids are missing something, so Hotpheex will refer you to a dwarf, Thordur, who is familiar with the ritual. Because he cannot leave the altar, you must visit him on his behalf. He will give you a disk of returning, and teleport you to the Dwarven Mines. Use the Activate option on the disk, NOT the Blackout option.