@HeirOfRick  
I was thinking something similar to soul-stealing, but less fatal to the victims.  
I think she stole parts of the magical essences of ponies, which would leave the victim weakened and looking somewhat emaciated (similar to what Tirek did in Twilight’s Kingdom, but to a lesser extent), though she would leave her victims strong enough that they could fully recover before too long.  
Originally, her victims would be bedridden for nearly a month, but after founding Princess Celestia’s School for Gifted Unicorns, she eventually could leave most victims strong enough that a full recovery would only take a few days.  
Of course she was disgusted by what she was doing to her citizens, hating herself for having hurt billions of little ponies over the years decades centuries millennium.  
But, because nopony had died (directly) because of what she’d done to them, she was able to keep convincing herself that seeing her sister again was worth the cost, even though she felt as if her very soul was becoming blackened and rotten.

@Background Pony #39BC
 
Superman, Darkseid is making deals with the mortals as Satan again!
 
“Goddammit, Darkseid! You can’t give me one month! Just one month!”

@Bhazor  
XD

@Yoshimon1  
Sold her soul to creature born before all known history.  
A creature that now walks free amongst the living, but is anything but.  
It’s name spawned evil. It’s breath spoke the first curse. It watched countless empires rise and gave rise to their destruction. It is not a creature of this world.  
It simply is…  
All hail Darkseid.

“I did 100 Push ups, 100 Sit ups, 100 Squats and a 10km run and now I’m immortal.”

@Background Pony #7697  
This.
 
I believe the Journal also says that they develop slower as well. As in: They were older than several adult ponies around them at the time, yet still didn’t have their Cutie Marks. And that was normal for Alicorns~

@Yoshimon1  

 
The Elements of Harmony are actually Celestia’s horcruxes.

 
Well… while there isn’t a detailed explanation for it, there is, as it so happens, something of an explanation in one of the official (supplemental) books to the series. Specifically, the Journal of the Two Sisters (written by former show writer Amy Keating Rogers). It says, “[Alicorns] age at a different rate than the rest of the pony races…”. So, yeah. They are just naturally long lived.

@Parallel Black  
Yep it’s very cool.
 
My take: obviously, Starswirl helped. (with whatever dark magic, but Celes wouldn’t opt for one that wasn’t some sort of self-harm I think.)

@Blissey1  
Well, especially in the case of depressed ponies, it could help them feel like their lives have meaning. Plus you’ve got to write what you love, right?
 
@HeirOfRick  
I thought we were going to go into a story arc, possibly a somewhat depressing one, but this is good too. Thank you for drawing this, I enjoy it :3

She’s a robot of course. :)

@HeirOfRick  
That’s true, soul sucking is overdone. And it definitely fits that she’d rather do things to herself than to other ponies.

@Yoshimon1  

 
…is it bad that I have an idea for a fic where celestia does this via vore. only willing sacrifices, though!

@Yoshimon1
 
Personally, I’ve never been a fan of the soul-sucking notion. It’s been done. And done and done and done. And I feel it would go against Celestia’s character to make other ponies pay for (what she perceives to be) her own mistake. When Celestia says she’d “do anything” I meant to imply she’d do anything to herself. I only had the vagueness idea of what that would be, but it was certainly very unpleasant. Ultimately though, I’d just like to leave it all up to the reader. Heaven knows I’ll probably never expand on this headcanon again.

@nitebeat  
A pointless gimmick that will never be useful to anyone.

but guys age spell…

@Tragiclady  
Generally, becoming a lich just means storing your own soul in a phylactery, though depending on the local laws of magic, the use of other souls may be a necessary part of the process.

@Yoshimon1  

 
I forgot if you had to do #1 in order to accomplish #2, but most of those sound like it would be a pretty interesting story.
 
If Celestia needed the lives or souls of other ponies to sustain her own, well those other ponies were probably boring anyways, and the means justify the ends, right?

@Yoshimon1  
First one sounds too obvious, third sounds a bit more plausible.

Alright, taking bets here. What’s the horrible thing she did?
 

There’s a lot of things I like about this

@Parallel Black  
Well, I always thought that Luna was reborn/recreated after Nightmare Moon was vaporized/purified by the Elements of Harmony rainbow friendship beam. And that’s why she seemed so small and helpless after the premiere.
 
I think the fanbase assumed that the normal alicorn immortality would kick in after that, since nobody said anything on the subject.

Huh. This is an interesting little thing. I don’t think I’ve seen the headcanon that Celestia is the only one of the two who is immortal.

How are either of you two still alive really? Alicorns, how do they work?