Once you enter Svartalfheim, you wont have access to the rest of the world until you leave. Bring things like food and bandages as they are not easy to find here without walking past agg mobs.
Certain rooms require fly or climb to get through, and one particular room will slay you if you're not flying. Bring a fly vial if you do not have perma fly or fly spell.
The entrance to Svartalfheim is in the Beowulf area.
You must go to the Prince Angantyr (2nw3sw2ne from The Pebbled Beach) and ask prince help.
He will ask if you'll repair his sword. Ask prince yes.
Go 2w2s2w from the prince and push wall to enter Svartalfheim.
WARNING: once you push wall you will enter Svartalfheim through a one way entrance.
After pushing the wall, you will enter an area with four maggots.
These maggots can randomly spawn, one at a time, upon entering a room.
Upon spawning, the maggot will try to bash you.
This first bash does not cause them skill lag and they may try again within two rounds.
After killing four maggots, the mother of maggots, Ymir's Abomination will spawn.
Find the swan-maiden (she is in an up room) and ask swan help. She will ask you to find her lost feather.
The feather will be spawned in one of four rooms:
Give feather swan and she will take you to The Barren Butte.
The swan-maiden will leave you in the main Svartalfheim area.
To the east of this room is the city of Joruvellir and the fields with the goats, Tanngnost and Tanngrisni.
To the north are the mountains leading to Mengloth and Utgard. (Blocked by a two-headed eagle)
To the west are the mountains leading to an alternative entrance to Utgard, near Nidhogg and Skoll. (Also blocked by a two-headed eagle)
To the south is Dvalin's Hall, where the Dvergar brothers Grer, Berling, Alfrik, Dvalin reside as well as the valkyrja.
To the immediate south west lies the troll area with Haugtrold.
In order to pass the two-headed eagles, you will need a runic blessing from Dvalin.
As you go through Svartalfheim, you will find spheres. You can give these spheres to Alfrik and he will give you a buff.
You can find a number of runes that will help you through Svartalfheim.