Tartarus, IMHO, would be Darkness, and here are my reasons why:
1. It's in the deepest part of the underworld (maybe not
deepest, but pretty deep).
2. It's in a pitch-black realm.
3. It takes strength from fallen creatures.
4. It's archetypically evil.
5. Darkness is awesome, and this would bolster concept.
6. Aside from pulling creatures towards it, I don't see Tartarus as Gravity.
Tartarus should rely on Death effects, the same as bonewall, so our favorite kitty cat should be all the more useful.
I get how Cronus is eternal, that making Time the ideal place to put it. I say, Cronus is obviously not a standalone card, and the cost is only for being rewound, eh?
Nice job using Percy Jackson to get your ideas, joe.
This idea is actually very useful.
EDIT: I'm torn about whether or not Tartarus should be Death, seeing as how it relies on destroyed cards. BTW, do spells that have been played count as destroyed?