This achievement is unlocked upon Defeating Sif, the great Grey Wolf in the Darkroot Garden.
You can get to this boss area by either buying the Crest of Artorias from Andre for 20k souls to open a locked door in the Darkroot garden OR by climbing up a ladder next to the large waterfall in the Darkroot Basin (near the Hydra). Either path will require you to traverse an area protected by several Forest Hunter covenant NPCs and then move through a stone structure where you can meet Alvina, an large feline that can invite you to join the Forest Hunter covenant. Beyond this structure is an open forest area that leads to the Sif encounter, which you trigger by entering an arena and approaching a large sword in front of a tombstone.
Sif is a giant wolf that wields an enormous sword in her mouth. She jumps around the area and swings the sword in large arcs. You want to try to stay underneath her where her sword attacks cannot hit you.
Upon defeating her you will receive the Covenant of Artorias (a ring that you must wear to engage the Four Kings in the Abyss) and the achievement will unlock.
As always, refer to
http://darksouls.wikidot.com for specific information relating to items or locations.