Im a very creative person, but when it comes to making stories im not. I feel disappointed in my ability to make bbeg and the stories. I know i want my story to have Something to do with Death as a Deity. Almost like the Deity from Vox Machina, but not. I want Death to somehow be connected to one of my players. And only that player can see them. But Somehow Death gets trapped or something, and he needs the help of the group or something. I think it would be a cool story line, but what i really need help with the the BBEG. How is death trapped or whatever. Whats the BBEG's Motive and such. 

Maybe another powerful creature is trying to ascend to godhood and take the place of death which weakens the deity. I personally would make death appear as a person a pc has lost in their backstory or maybe even appears as whatever god the pc follows. 

I could do something along the lines of Deaths physical form has been killed, so his "soul" or whatever death has, went to one of my party members and at first hes almost like a parasite, but the pc realizes whats happening? 

There is definitely a cool idea there. You can probably go antagonistic if the pc realizes what is happening or have them be deceived for a while. That sets up a really cool plot twist down the road.

Or perhaps someone tried to escape death and when Death went to claim the soul, they had trapped him in a like a necklace or something that allows whoever has it to see death. And his motive is to be freed since the only way to escape is to kill the creature that trapped him, thus he constantly advocates the players to go after the creature. Idk, just ideas you do you.

I think the first thing that might help is ask why you want this deity to have a connection with a player.

It sounds like you might want the player to feel a meaningful connection to this deity, and then later be surprised by a revelation or event.  Which would be, of course, awesome.  An easy way to do this is to make the deity an NPC they form a Bond with.  Death is out of balance (maybe they're trapped in the Ethereal Plane like in Ravenloft, or perhaps the deity's portfolio has been stolen by other powers).  They request the heroes accomplish tasks, maybe even travelling along with them and making friends (a la Gandalf).  And then, the grand finale, in order for this realm to be saved, you must destroy me and take the mantle of death, and save this existence.

So, in the Comics themselves, Thanos fell in Love with Death.

The events that the films drew from were based on his actions to make Death Happy -- half of all life was meant as a present for his girlfriend.

Death was captured by one of the celestials, I think it was (been a really long time, lol) and thanos had to go after her.

That said, for your BBEG, they need to be something that is "outside Death's Purview" -- so an undead, like a Lich or a Vampire, probably wanting Death to restore them but not to life, jst to a decent body without the whole decrepit thing or weakness to sunlight. Maybe an immortal demigod (for that which is immortal is not mortal, and Death only has call over that which is mortal).

Perhaps they summoned Death using  arcane arts and Death is trapped in a cave, still within the magical circle, surrounded by all manner of hazards, but of course the BBEG doesn't know that Death has a Champion. Or perhaps the BBEG wants to be that champion. Or maybe the BBEG is courting Death, like Thanos did.

THey will be powerful, capable of seriously damaging the party in any given encounter, but they will also be cowardly, and will work to escape before they get cornered, at which point they will fight with everything they have.

Because while they may be immortal, they can still be destroyed. Being destroyed is not being killed, not dying -- dying means there is an afterlife (probably the Abyss for this one) but being destroyed means they will not be and all they have wrought will fade.

not much, but something.