Eren Yeager is so powerful because he wields the Attack Titan, the Founding Titan, and the War Hammer Titan, combining their unique abilities like future memory manipulation, mass titan control, hardening, and immense regeneration, all amplified by his extreme drive and experience, culminating in the devastating power to trigger the Rumbling.
Eren possessed the power of three Titans. From his father, Eren inherited the Attack and Founding Titans. After eating Lara Tybur during the Raid on Liberio, he gained the War Hammer Titan as well.
After the end of the manga, we learnt that Eren killed 80% of humanity and the 20% of it mostly lived in Paradise Island. What was believed to be a lie at the start, turned out to be a reality in the end.
Eren does not have royal blood. To have it you'd have to be descended from the Eldian royal family. Zeke has it through his mother Dina and Historia through the Reiss family. The Founding Titan should be able to control any Titan, regardless of whether the Titan has royal blood.
While the results were inconclusive at the time, King would later learn he is a young Titan. It is unknown for sure due to his age if King's blood has the exact same properties as that from a fully-grown Titan, though he did briefly question if his blood was needed in freeing the Collector.
Fritz (フリッツ Furittsu?) is the former king of the Walls; however, he was a puppet king who acted as a substitute for Rod Reiss, the real king at the time. After the coup d'état and Rod's death, he was replaced by the real ruler of the Walls, Historia Reiss. He is unrelated to the real Fritz family.
While initially presented as a heroic protagonist, Eren gradually becomes more of an extremistic anti-villain by committing several violent acts and global genocide, for what he claims to be for the sake of Eldia, at which point he is focused on less, which causes most of his friends to turn against him.
While it has not been performed by every Titan that has been shown, it is noted in the series that all Titans can harden with enough practice and training. Hardened materials on Titans were shown to be penetrated by Thunder Spears, sheer strength (presumably by another Titan), or the power of the Founding Titan.
In season two, Eren Yeager and Armin Arlert, among other Survey Corps members, began growing suspicious of two of their comrades, Reiner Braun and Bertholdt Hoover. Reiner and Bertholdt eventually confessed the truth to Eren, admitting they were not just Marleyans undercover, but Titan shifters too.
The bloodline is still intact. The Hallucigenia (the source of all Titan powers) isn't even dead — it crawls out of Eren's head after the final fight, still alive. The foundation for the Titan system is still very much in place. The only thing that's changed is that Ymir chose to disable it.
Approximately 80% of all human life on the planet was ended by the time the Rumbling was stopped prematurely by the combined efforts of the remnants of Paradis Island's Survey Corps and Marley's Warrior Unit.
Because it was impossible for anyone to surpass the Founder, each person who gained the power of the Titans was fated with the "Curse of Ymir" (ユミルの呪い Yumiru no Noroi?), which limited their remaining lifespan to only 13 years after first acquiring it.
However, if the Founding Titan is inherited by someone outside of the royal family, the power can still be used if the inheritor is in physical contact with a Titan with royal blood. This is demonstrated when Eren Jaeger temporarily used the Founder's power after striking the hand of Dina Fritz's Pure Titan.
The Cart Titan is in no way a weak link, but it can be considered the weakest of the Nine. Held by Pieck Finger for the run of the show, the Cart isn't built for direct combat or known for its overpowering strength.
Immunity to the Founding Titan
The Ackermans are former Subjects of Ymir and could manifest within Paths; however, they were immune to the Founding Titan's ability to alter memories. This led Kenny's grandfather to speculate that they were not Subjects of Ymir as the majority race within the Walls were affected.
They were called the Founding Titan, the Colossal Titan, the Female Titan, the Beast Titan, the Armored Titan, the Attack Titan, the Jaw Titan, the Cart Titan and the War Hammer Titan.
As other people have said, the actual power is contained within the spinal fluid of the shifter. If the Santa Titan had chewed, it would have been game over for Eren.
Zeke Yeager, otherwise known as the Beast Titan or just The Beast, is the main antagonist of the Attack on Titan franchise. He is the son of Grisha Yeager and Dina Fritz, who had him join the Marleyan military for their own agenda.
In the manga, Eren didn't know why he destroyed humanity at all. He just knew he had to, because he got the memories from the manga.It turns out he was a slave to himself.In the manga, he just wanted to destroy people. He just wanted no people across the ocean.
Eren's true love, widely considered by fans and supported by the creator, is Mikasa Ackerman, though their relationship is portrayed as tragic and complex, evolving from a sibling-like bond to deep, albeit complicated, romantic love that he struggles to acknowledge due to his grand, world-altering plans. He confesses his feelings in the end, but it's within a tragic context, acknowledging he couldn't bear seeing her with anyone else and seeing a future where they lived happily in an alternate reality.
Rod's titan form also lacked a face. It had been destroyed by the sheer friction of him dragging it along the ground as he moved. The damage was so great that even the titan's brains were exposed.
In an interview, Hajime Isayama mentioned that out of all the veterans of the Survey Corps, Moblit drinks the most due to his "unfortunate" position.
Not only their names but their fates were also the same. Looks like Isyama had already given the spoiler of Ymir Fritz's story through Ymir. Except for one fate after getting Titan power, Ancestor Ymir fell into submission under King Fritz and married him and was a slave to his bidding to the end.