So I am attempting to create a Ghostrider themed subclass for the warlock. I am using the Hexblade as a base and adjusting from there. The problem I am having is trying to figure where exactly I would put the "Hellfire Chain." I was thinking about it being similar to a pact/hex weapon but I can't seem to get it working on the sheet properly. Any help would be greatly appreciated as it is my first homebrew subclass. Thanks in advance.

I don’t know if this will help you at all, but one of the first PCs I ever played in 5e was a Ghostrider inspired character. I made her as a Paladin and had permission to combine elements of the Oath of Vengeance and Oath of Conquest subclasses. The find steed / find greater steed spells stood in for summoning her “bike.” For her chain I reskinned a halberd to do bludgeoning damage, and had permission to reskin spells like conjure barrage and spiritual weapon for the crazy stuff she could do with her chain. She was also a Fallen Aasimar, and had the Haunted One background. It was awesome. 

 If any of that helps you at all, even just for some ideas for Patron spells, rock socks with it. 🤘 

Oh, by the way, to get the ability to designate a weapon to use Cha instead of Str/Dex, you have to add a feature named “Hex Warrior” to the subclass. It’s hard-coded to the feature name.