Eren was controlling him and all the other past shifters and the reason he used bertholdt as the colossal titan to do the job even though there were past colossal titan shifters he wanted to give everyone who was close with bertholdt hell by using him.
The one thing that doesn't make sense is that Eren lost control of the rumbling when Zeke died. Cause previously everyone just assumed he needed Zeke in order to control Ymir, but afterwards she gave Eren control and therefor didn't really need Zeke for it anymore. Unless of course we separate the worm entity and Ymir into 2 separate beings with their own intentions. But just cause Ymir suddenly doesn't let Eren control the collossals anymore doesn't mean she can override the worm's intentions, no? Got a little clouded at the end there.
the titan that eat the eren we can see him.in the rumbling when reiner is running from 3 titan we can see the titan that eat the eren.hes stomach was so big
