Story related, cannot be missed. You will get this after defeating Sif, the Great Grey Wolf. Before you can fight this boss, you need to get the Crest of Artorias from Andre the Blacksmith for 20000 souls. Then go into Darkroot Garden and open the door (be sure to hit the wall to the left for a bonfire). Then traverse the area and get to Sif’s boss fog.
Sif isn’t the hardest fight but it can still be tricky if you aren’t used to dodging. Just stay underneath him and attack while you're under him. Be sure to let your stamina to recover and heal when needed.