So, I have seen so much talk of this atma business. I am starting to understand more about end-game ... ilvl, hard modes, and whatnot, but after searching pretty extensively, I am unable to get an actual definition or basic guide as to what atma actually is. I know bits and pieces about it but overall, I have a lot of questions. Seems like it's an item(?) that there are 12 of that involve grinding fates ... Please explain?

a) what is Atma?
b) What does it actually do/what's it for? Held item? Equippable?
c) how do I go about actually getting them? I know it involves fates, but is there anything else to it? Which fates? Any of them?
d) Why break my back grinding Fates to get them? Are they that important?
e) Is there any kind of minimum ilvl or quest prerequisite?

Thank you for any help.