"N̈", or "n̈" (referred to as n-diaeresis or n-umlaut) is a grapheme from several minor extended Latin alphabets, the letter N with a diaeresis mark.
It occurs in the orthographies of Jacaltec (a Mayan dialect), Malagasy, and Cape Verdean Creole, in all three cases representing a velar [ŋ].
The "N̈" appears in very few languages, and is thus not represented on any computer keyboards in any languages. It is also not available as an HTML entity.
Neither "N̈" nor "n̈" are characters in the Unicode character set, and have to be represented as a letter "N" (or "n") and combining diaeresis U+0308.
In popular culture
The letter is probably most commonly known for its use in the title of the fictional band Spın̈al Tap. It is used as a parody of the heavy metal umlaut used gratuitously by several actual bands, such as Motörhead, Mötley Crüe and Blue Öyster Cult. The video game Borderlands 2 contains a boss named Captain̈ Flyn̈t.