The Crest of Artorias is one of the most recognizable items in the original Dark Souls game. The item is used to unlock the gate in the Darkroot Garden which gives you access to the Sif Boss fight. This item is originally sold by Andre for 20,000 Souls or dropped by Andre if you decide to kill him. Here is where you can find the Crest of Artorias in the Dark Souls: Nightfall Demo.
Going beyond the lava pools and making your way up to the Darkroot Garden, you will need to head to the left. Follow the path until you reach some new enemies with long arms. Dark Souls veterans will recognize this area as the part of the Darkroot Gardens before the Moonlight Butterfly boss fight. Reach the other side of this forest area and enter the tower on the other side. You can either run past the enemies here or fight them. If you decide to fight, we recommend taking them on one at a time since it is easy to get overwhelmed here.
Once you reach the tower, head up the stairs to where the Moonlight Butterfly boss fight would normally take place. Be careful of the stone golem at the top of the stairs. Make your way across the bridge to the tower on the other side. Go to the top of the tower. Watch out for the magic-wielding enemy at the top of the tower. Take them out and there will be a chest and an item next to where they were. This is where you will find the Crest of Artorias.