Memory of Jeigh

Travel to the Cardinal Tower bonfire. Climb down the ladder and cross the bridge. Go through the door and head outside to find the massive door. Open it and light up the The Place Unbeknownst bonfire inside. Head outside and drop off the stairs. Loot the corpse hanging by the wooden ledge to get Twinkling Titanite x2.

With the Ashen Mist the dragon gave you, you should be able to interact with the tree-like remains of a giant. This will transport you to another area or timeline, rather.

As soon as you get transported, you have no choice but to enter the mistry doorway and you'll find yourself in the middle of a battle. Take note of the spots where the bombardment lands; you can take immense damage if you stay on those spots for too long. Go upstairs to the left and you'll find a corpse containing a Bonfire Ascetic. Take out the two giants and be careful of the giant statue head that will come rolling down. At the end of the path, you'll find the Giant Lord.

Boss: Giant Lord

The best way to deal with this boss is by fighting it up close. That way, you don't have to worry about its massive sword. You just need to look out for its stomp and sword swings. Keep moving around, preferably behind it and anticipate the direction of its next attack.

There's a corpse sitting by the wall containing an Old Radiant Lifegem and another one after the stairs containing a Fire Seed. Go inside the doorway and examine a giant's corpse to get an item. You'll be instantly transported outside the memory after staying inside for a few minutes.

You still need to find another giant's remains and interact with it to revisit another memory. Travel to the Cardinal Tower bonfire. If you've blown up the wall in the Cardinal Tower from the other side, you can go through that hole, go down the ladder and find the tree-like giant's remains. If not, you have to drop down to the lower floor from the bonfire and go past the opened gate leading to the corridor with the giant statue sword, then turn right to the next area.