For most of Nightcrawler's existence, they've played it as figurative, but at least through JB's era, he could (when the script called for it) literally disappear in shadow.

Which, come to think of it, would have been a lot more plausible without the red circus outfit that he wore when fighting evil. A dark blue or all-black costume would've made more sense for a character who becomes completely invisible in shadow, but that's unstable molecules for you.

Not that I'd have changed Nightcrawler's costume a bit from Dave Cockrum's original design. Even minor tweaks to that costume have looked awful, and I can't think of a single Nightcrawler costume that I've liked remotely as much as Cockrum's original. Ditto for Storm and Colossus.

(For some reason I'm fine with the Sub-Mariner's many, rarely used abilities resurfacing though.)

Arc -- not trying to be snarky, here, but do you ever read ahead to see if a question has already been answered before you post your own response?

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That's OK John. I apologize for it .

Now I feel like you are watching over me...