Comments of chapter undefined of Abyss Mage

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 :)