Webnovel Author: Juarez_Culau - Novel Collection

He has the tendency to follow the will of the other Zelhart's because he is since the beginning becoming one with Darkness. He lived his own life what made him different, that's why he has two names in the novel. To summarize, as a start he was someone totally new, shaped by his own environment, as Robert lessons and Scullery betrayal, that is Gremory and he was becoming one with Darkness this whole time, transitioning from Gremory to Zelhart, but still with traces of Gremory. So yes, he can break away, since he is Zelhart but still have the same will as Gremory. Further in the novel, there will be more information about this particular matter that may clarify any other question, if you already read that and is still confused, send me a message on discord or leave a comment at that chapter and I will be able to answer without giving a spoiler.

I am not sure if I understood the question, but I assume you asked about his essence that will still be alive after his death. So just to remind you of some small details, Zelhart and Hulum Ying are basically good and evil, think as something material, manifestation in the air and also affecting his surroundings, people, and so on. You may remember how the people close to Zelhart felt anger and rage, giving them strength as what happened to Claire. What I just said doesn't answer your question, and I know that, think as a clue; I can't explain further than that right now, but maybe it's enough for you to deduct what makes him Zelhart. If not, then just wait a bit, and the story will tell you :)

In short: Because the Giant Worm was an enemy that he absorbed. Full explanation: Mana and dark energy are two different types of energy that he can convert to many uses, he absorbs dark energy from the enemies he defeats, and as the process of hypertrophy, it stimulates his body, "Elementary Particles" which enable him to evolve when he absorbed enough, as someone training at a gym and creating muscle through hypertrophy, but here the major difference only occur when he reaches the breaking point. That said, when he creates an undead, he uses his own mana. For that reason, he can absorb back from his undead, or even a mitre crystal if it's filled with mana. There are some exceptions when he can acquire mana from other sources, as when the Oracle shared her mana with him, but I think this answers your question. If not, you can call me on discord, and I explain it better :)

Yeah, I am not sure if I keep doing these same mistakes but I will keep that in mind. Thank you :)

Thanks for reading so far. I'll try and hope you enjoy :)