Black Clover has revealed that while Asta's Devil arm upgrade granted him a temporary boost in strength, it comes with dangerous side-effect. As Asta and Yami worked together to defeat Dante of the Spade Kingdom's Dark Triad, Asta reached a new level of understanding with the Devil inside of him and got it to lend him more power. The Devil accepted this bargain at the cost of a piece of Asta's body, and this revealed a much different right arm for Asta as he was able to deal the final blow to Dante. But now that the intensity has winded down, Asta's received a dangerous prognosis.
As the fight winds down and Asta and the other Black Bulls prepare for the next step in the latest few chapters of the series, it was revealed that Asta's Devil arm no longer fades away like when he used the power before. Instead of disappearing along with the rest of his Weg attributes when he uses his anti-magic, Asta's Devil right arm is now permanent as none of the medical mages available can do anything about it.
When the Devil agreed to lend Asta more power, it stated that Asta's body was at its limits and thus needed to take something from Asta in order to push more of his power into him. This ended up being Asta's right arm as we see shortly thereafter, but it only buys Asta a matter of seconds before his body reached its limits once more. But soon Asta realizes that this power was barely enough to defeat Dante, and nowhere near enough to save Yami from being taken away by Zenon.
Thankfully, the newest chapter of the series has provided some hope with the Black Bulls' Vice Captain, Nacht. Appearing for the first time in the series, Nacht revealed that he was not only another Devil host like Asta but is willing to teach Asta how to effectively use his Devil's power. So even though Asta's body is permanently changed, there is a way to keep it from changing further.
Asta gave up his arm in desperation, and hopefully his training to use more of his power will keep him from sacrificing more of his body in the heat of the moment. But what do you think? Does Asta's now permanent Devil arm mean he's closer to a full takeover? Will he be able to fix it someday? Let us know your thoughts in the comments or you can even reach out to me directly about all things animated and other cool stuff @Valdezology on Twitter!