Nacht was definitely one of the surprise debuts during Black Clover's recent arc, and the newest chapter of the series continues with those surprises with a full look at his devil form. After returning from his undercover mission in the Spade Kingdom when Yami was kidnapped by the Dark Triad, he has taken Asta under his wing in order to train him how to use his devil's power in a more efficient way. We've been seeing more of his abilities with each new chapter of the series, and the newest one takes this one step further.
The Black Bulls' Vice Captain has played things fairly close to his chest in his appearances so far as it's been made clear how distrusting he is of the Clover Kingdom as a whole. But he's starting to bring these walls down little by little with Asta, and the newest chapter revealed his full Devil transformation as he prepares to fully begin his training with Asta after Asta successfully completed the Devil-Binding Ritual.
Chapter 270 of Black Clover sees Asta successfully defeat his devil as part of the Devil-Binding Ritual, and Nacht tells him to make a contract on a feeling of total dominance to completely command the devil's power without chance of refusal. Asta refuses and instead befriends his devil, and Nacht begins to smile slightly while joking with his own devils that Asta is an idiot.
Nacht then looks happy and reveals that Asta made the "right" decision in choosing to work with his devil as equals (teasing that Nacht has the same kind of contracts with his own devils), and thus vows to begin training both Asta and his devil going forward. Summoning one his devils to his side, it covers him and reveals his full Devil transformation that includes a long black cloak, hat, gloves, and even boots coated in shadow.
Chapter 270 then ends on the cliffhanger with Nacht telling both Asta and his devil to attack him and defeat him, and so we'll be seeing more of what Nacht can do with this form soon enough. We have only seen glimpses of his power and personality in the previous chapters, but it's clear that he'll be letting his guard down the more he works with Asta.