Natsu sucked up the black fire after emptying his magic reserves. Ben's clone and Zancrow were surprised to see this. Ben's clone gained an understanding of how Natsu could do that while Zancrow didn't understand at all.
As a Dragon Slayer, Ben's clone understands how Natsu's magic worked. When Natsu ate the black fire, he came to a realization why Natsu was able to eat it just now. Natsu might have done it out of instinct, but Ben's clone knew the real reason.
'The God Slayer's flame is a tier above a Dragon Slayer's flame, that's true. But that's not the reason Natsu couldn't eat it before. It's because Natsu still had his magic power before, so the consumption didn't go well.' Ben's clone thought.
When eating elements, a Dragon Slayer and other Slayers transform their element into magic power. The new magic power will match their Slayer Magic and become their magic power. But lower-tier Slayer mages can't eat the higher tier because they can't transform the magic power to become theirs.
It's like using better fuel for a car when you usually use cheap fuel. That is good but it also tends to cause trouble for the machine and it's not effective. The solution is to clean the fuel tank and all the machine parts before changing the fuel type.
Well, that's the best comparison Ben's clone can come up with. The main point is that a Dragon Slayer can't absorb a God Slayer's magic when they still have their magic inside of their body. Both types will clash and God Slayer Magic will win because of it's higher tier.
But when the magic storage is empty, the God Slayer's magic won't clash with anything and the Dragon Slayer's body will be able to absorb it. Their body will adapt to the God Slayer's magic and absorb it faster because there's no clash of power.
Even so, Ben's clone also thought that it also depends on their strength. He had a hunch that he could eat the black fire without emptying his magic power reserve. It's because his body and magic power were a lot stronger than Natsu so he should be able to consume the black fire immediately. But he needed to test it to make sure of this.
Ben's clone intercepted Natsu's battle to test it before Natsu defeats Zancrow. Now that Natsu knew how to handle Zancrow, he should be able to win. So Ben's clone needed to do it now or he won't get another chance.
"What are you doing Ben?" Natsu asked as Ben stood in front of him.
"Sorry, I just want to test something. It won't take too long, so just wait a moment." Ben's clone said.
He walked towards Zancrow and taunted the guy to attack him. Zancrow was irritated and attacked Ben's clone by shooting black fire from his hand. Ben's clone just took it and was surprised that he was hurt by the fire.
But it wasn't as painful as he thought after seeing Natsu be hit by it multiple times. It's just slightly hot and painful, like holding a glass filled with hot water. This is still bearable and not something that will kill him.
So then he tries to eat the fire before it disappeared and his hunch was right. He can eat the black fire although it wasn't very pleasant. It's like eating a hot and spicy soup on a very hot day. His mouth and stomach hurt a little, but he can eat it.
"Heh, so this is God Slayer Magic, huh? Not bad at all. Maybe if you are stronger, just a little bit close to my strength, then maybe I wouldn't be able to eat your fire. In the end, your power matters even if your magic is considered to be better. I'm done, Natsu. You can finish this." Ben's clone said as he walked away.
"Oi, bastard! Wait!" Zancrow angrily shouted because his attack didn't work on Ben's clone and he was looked down on.
But he doesn't have time to pursue the clone because Natsu had prepared an attack. Natsu had his right hand covered in orange fire while the left hand was covered in black fire. Then he used both types of fire to attack Zancrow.
Zancrow took it, assuming it wouldn't affect him but he was wrong. The combination of both types of fire isn't something that he can take. Besides, Ben once told Natsu that an immunity has it's limit and the combination of fire surpassed the limit of Zancrow's immunity.
This attack defeated Zancrow and Natsu left after Ben gave him some fire to eat. Ben's clone stayed behind and waited until Natsu was gone. After Natsu left, Ben approached Zancrow and put a handcuff made out of Magic Sealing Stone on him.
"You will tell me everything you know about God Slayer Magic." Ben's clone said, smiling wickedly.
Ben wasn't heartless, but he didn't care about a evil man's well being. Zancrow is a dark mage that killed a lot of people so Ben wouldn't hesitate to kill him if it was necessary. But now he needed Zancrow to tell him about God Slayer Magic, so he wasn't going to kill him.
Ben's clone took Zancrow to the top of the Tenrou tree. There wasn't anyone else present, so he could do whatever he wanted to Zancrow. He will extract information from Zancrow even if he had to torture the guy.
He wasn't fond of torturing people, but he was trained to do it as a soldier in his past life. Well, he wasn't trained to torture his target, but he is trained to handle torture to avoid sharing classified information. That training taught him how to torture someone though.
"It would be good if you told me everything you know so I don't need to use violence. You should know I'm not a pure-hearted man." Ben's clone said before he began to interrogate Zancrow.
Elsewhere...
Elfman and Evergreen found themselves contending with Rustyrose. It was a difficult battle for the two because their opponent used a very versatile magic known as Arc of Embodiment.
This magic allows it's user to create anything from their imagination as long as they have sufficient magic power. That means he can make anything from nothing. He can also make something based on his opponent's magic to counter it perfectly.
His magic isn't limited to non-living things. He can even make a living being, just like the Black Arts. Furthermore, he had a lot of magic power and can create strong things to counter Elfman and Evergreen's attacks.
Rustyrose created 2 sacred guardian beasts: Belcusas and Belfast. Both beasts were enormous and they are strong. Belcusas was a heavily armored man-like beast while Belfast was a wyvern-like beast.
Previously, Rustyrose only used Belcusas, but Elfman was too strong for Belcusas. Ben and Mira trained him extensively, so it would be disappointing if he can't fight one beast like Belcusas. Also, Evergreen was there as well, so fighting one beast wasn't a challenge.
Elfman and Evergreen did a lot of damage to Belcusas so Rustyrose summoned Belfast. He also aided his guardian beasts by creating a beast-like arm. It was similar to Elfman's partial Take-Over, but it was a result of the Arc of Embodiment.
Right now, Elfman fought the 2 beasts while Evergreen tried to hold Rustyrose in place. It wasn't easy and the two were pushed back. Elfman was in his Beast Soul form, but this wasn't very easy.
It was a lot harder for Evergreen because she had a strong opponent. Her petrification magic was useless because Rustyrose kept making glasses to block it. She received a heavy blow and was pinned down.
After seeing this, Elfman became angry because Evergreen was his precious guild mate. He was suddenly attacked by Belcusas but he blocked it with one hand. Then he suddenly stabbed Belcusas in the chest with his spear-like claws.
Rustyrose was surprised that Belcusas had been defeated by the angry Elfman. Then it was taken to a new level when Elfman transformed into Belcusas before it disappeared as it was just a magic construct.
"Unbelievable! He took over my beast." Rustyrose said with widened eyes.