u/Creepy-Hold-

I finished watching attack on titan a while ago and have been trying to find an answers to my questions unsuccessfully.

To my understanding the hallucigenia is an autonomous being driven by the desire to survive and multiply, acting like a parasite.

Why is Ymir able to control it? Why is she able to give Eren access to the founding titans full power and why is she able to then ultimately destroy the worm just by seemingly deciding to? I thought Ymir was more the one who would be controlled by hallucigenia if it acts like a parasite? why would the worm just disappear???