Board Thread:Theories/@comment-32443384-20180201102915

Short: only the royal bloodline can use Multiple Titan Shifter Forms at once, the Founding Titan IS Multiple Titan Shifter Forms and that is why Eren cannot use it, because he can use only one Titan form at once and that is the Attack Titan because he is not of royal blood. The founding titan could split into dozens, hundreds, of titan shifters by the end of the story.

Long:

Shifter Lineage: to become a shifter a human must be injected with a titans’ spinal fluid which triggers transformation automatically, or injected with the spinal fluid of a titan shifter of royal blood and then triggered into transforming by the scream of that shifter, then the titan form must devour a current titan shifter, the result is that the titan returns to their human form and their titan form is replaced by the new titan shifter form, of which there are nine, although the human currently ‘piloting’ the titan form drastically changes the look of the titan form to appear somewhat more like themselves. It is possible for a titan shifter to devour another titan shifter and possess two titan forms simultaneously, as shown by Grisha Yaeger who first attained the Attack Titan form and then devoured the Founding Titan shifter and attained that form although he was unable to use the second form as he was not of royal blood. It is also possible for multiple titans to be passed down to the next titan who would devour such a shifter, as shown by Eren Yaeger who devoured his father, Grisha, while in titan form and attained both titan forms at once, although Eren also had no royal blood and could not use the founding titan form.

As a failsafe to maintain the titans’ lineage a shifter killed but not devoured will have their titan assigned to a random newborn infant in the world, in this way a titan shifter can never be lost to death. A titan shifter can only be ‘lost’ by merging with another titan shifter as above. It is likely that if a titan shifter holding multiple titans dies and is not eaten, their titans would again be split into multiple random infants throughout the world, but this is not confirmed. Regardless, every titan shifter has been passed down by devouring and not random selection for at least the last 100 years. This includes the very powerful Warhammer Titan and Founding Titan, the founding titan in particular has been passed down by the royal families devouring ritual for possibly nearly two thousand years.

There are currently eight titan shifters, the Founding and Attack Titans being merged into one, there were previously nine. With the above information, it is possible that there were previously many more titan shifters, that they were merged by devouring into only nine, and kept that way by the devouring rituals of the shifters that held them. That the Armored, Cart, Colossal, Beast, Attack, Female, and Jaws have been jumped randomly between bloodlines and even taken over by Marley (excluding attack), to be jumped between brainwashed youth as weapons of war, leads one to believe that they are singular titans and not merged. That the Warhammer and Founding Titans are so powerful and have been passed along the same family bloodlines for generations, in the case of the founding titans for hundreds of generations, leads one to believe that they may infact be multiple titan shifters hosted by a single shifter.

It may be that only those of royal bloodlines can effectively use the power of multiple titans, as shown in Eren being unable to use the Founding Titan, the Founding Titan may only be so powerful because it is merged and that’s why only the royal bloodline can use it. It may also be that the more titan shifters are merged into a single shifter, the more human-like the titan form becomes. It could be that if all the eight current shifters are merged, along with the unknown number of shifters possibly within the founding titan, the titan form would be almost perfectly human with a complete digestive tract, and I imagine it colossal sized 60m tall.

Edit: Alternately, the founding titan could be again split into its multiple titan shifter forms and result in dozens, hundreds, of titan shifters, by the end of the story. 