Valhalla Ruins is a weekly 6-man instance where you fight five MINI/MVP's for a chance at their drops.
Once you reach base level 60 this will unlock in the Instance menu.
Location of Valhalla Ruins in the Instance menu
The instance menu offers offers 6 stages across 2 difficulties. Each stage and difficulty provides access to different MVP/MINI bosses every week. Select each stage to review the available bosses and choose the one that drops cards you're looking for or the one that may drop stuff worth the most zeny since higher difficulty here does not mean better rewards. Most equipment is dirt cheap except for gear used by popular jobs.
You can use the arrows to the sides of the MVP/MINI boss model on the right side of the screen to see each of the bosses that you'll encounter at that difficulty/stage.
Example of Elite stage 100 with Dark Lord as one of the bosses
Example of Elite stage 170 with Gem Spirit as one of the bosses
This instance takes place on a map where MVP/MINI bosses spawn in the same 5 locations on the map shown below. These bosses are significantly easier than the world boss versions and give everyone in the party loot rather than just the MVP so this is a great way to target bosses for specific loot.
You can also spend lucky coins after any boss is killed to make a second roll on its loot table. Personally I would save those coins for ancient equipment but later on once you know what you're doing you may want to do this.
If you die you respawn at the entrance of the ruins and can try again. You can leave at anytime and try again later in the week too so if you can't do all five bosses in one go.