Just as you get attached to a character in Crusader Kings 2, what happens? They die—of illness, of old age, of stress or depression—and you're onto the next. Now you can limit the chances of that happening, and add a bunch of new powers, with Gods! It's a mod that lets you play CK2 as, well, a god.
The Gods! mod lets you become a living deity on earth. You're immortal, thought that doesn't mean you can't be killed, just that you won't die naturally. For instance, at one point I fell ill, but then my godliness kicked in, instantly saving me from pnemonia. You can still be assassinated or killed in battle, however, so you'll need to take care not to make too many enemies, or at least make sure your levies are larger than those of your rivals.
Being a god, good or evil, has some other perks as well, including special buildings and military units. For instance, you can create angels (or dark angels, if you're evil), provided you've met certain milestones of prestige and goodness/darkness (which is the modded version of piety). These angels appear in your court as babies, but once grown you can assign them to lead your armies and retinues on the battlefield.
One of the new god powers is being able to whip up a baby without having to, y'know, actually get busy (see also: The Bible). You can choose to create a demigod through divine conception: magically impregnating someone who then gives birth to a demigod. If you're playing as a female god, it works the same way only you're the one who carries the godly baby to term. This child carries around some of the powers of your bloodline, and can pass them on to their own children.
You'll have some new titles to bestow upon your god-children, too, like God of War or God of the Dead, depending on their stats. Just be careful about creating too many magical godbabies. Just like in the vanilla game, family members can get jealous when it comes to what they stand to inherit, and you probably don't want a bunch of demigods out there plotting against you.
I know that being a god seems like it would strip any challenge from the game, but quite honestly it's still pretty tough playing as a god. It's still the same game where you still have the same problems (though I played as evil god and so pretty much everyone hated me until I threw them into prison and forced them to adopt evil as their religion). As a god on earth, you still have to deal with people, and dealing with people isn't any easier just because you can magically create babies or fight off the flu.
To become a god in the first place, you can choose it using the game's ruler creator, or just let it happen in-game. You'll begin having strange dreams, at which point a seer will visit you and ask if you'd like to be trained to become a god. I'd suggest you say yes.
There are a bunch more features you can read about on the mod's Workshop page, where you can also subscribe to it.