What do I do with the Dragonrider soul?
People also ask, what do I do with the pursuer soul?
Soul of the Pursuer, who lurks in Drangleic. The Pursuer, who seeks the bearer of the sign, will not rest until his target is slain. Use the special soul of the Pursuer to acquire numerous souls, or to create something of great worth." Soul of the Pursuer is a consumable item in Dark Souls 2.
Thereof, what do I do with the soul of the last giant?
The Soul of the Last Giant is acquired by defeating the first boss in Dark Souls II, namely The Last Giant. This soul is required to create the Giant Stone Axe. When the soul is used instead it gives you 6000 souls. This removes the soul from your inventory, which means you can't create the axe any longer.
The Dragonslayer soul creates Ornstein's Ranseur (Dragonslayer Spear). While this evidence can be used to further the idea that the Old Dragonslayer is Ornstein, but fallen to Dark, it could also mean that it is the same soul that has taken on a new form, like the other Old souls.