Spoilers of season 3 part 2 ahead do not proceed to read if you haven’t yet watched that far yet!
A little Background
As the story progresses, we learn humanity beyond the walls is not extinct, and that the people living inside the walls are the descendants of a race of people called ‘Eldians,’ i.e. the people of the kingdom of Eldia, who lived freely centuries ago.
From these people a woman named ‘Ymir’ somehow gained the power of titans, no one knows how. But in doing so she cursed all Eldians to becoming humongous titans, given that they were injected with a certain spinal fluid. There are mixed opinion to if she was benevolent and kind using these powers or if she was cruel and wreaked havoc. Anyways, she died exactly thirteen years after she attained these powers, and before dying, split her powers into nine different special titans, each with its own name and separate power, physique and owner.
The Nine Titans
Any Eldian injected with the spinal fluid of a titan becomes a mindless titan, and cannot revert back to human self. However, the Eldians chosen to hold the power of one of the nine titans are conscious even while they’re using their titan abilities. They can become a human again.
There is an Attack titan, Colossus, Armored, War-hammer, Beast, Female, Jaw, Cart-man and the Founding Titan. The Founding titan, also known as the ‘Co-ordinate,’ is the most powerful titan as it holds the power to control all other titans and erase the memory of people by Screaming. However, the power of these titans weren’t just given to random Eldians, these were handed down in the royal families within the kingdom of Eldia. Also, the Founding titan is the only titan out of the nine whose full power and potential cannot be awoken or utilized if the user is not of Royal blood.
Inheritance of the Nine Titans
There is a rule to how the power of the nine works however. Any user, after attaining the power of any of the Nine titans will die exactly 13 years afterwards, since Ymir herself wasn’t able to keep the power for any longer than that.
There are two ways by which an Eldian can attain the power of the Nine titans. The Eldian gets injected with the spinal fluid and turns into a mindless titan eats one of the Nine titans and attains the power. OR, the current user dies without passing off his power and the power transfers randomly to the next infant who is born after the user’s death. Also, if one of the Nine titans eats another one of the Nine, it gains the power of the one eaten.