N̈
"N̈", or "n̈" (referred to as n-diaeresis) is grapheme from several minor extended Latin alphabets, the letter N with a diaeresis mark.
It occurs in the orthographies of Jacaltec (a Mayan dialect), and Malagasy, in both cases representing /ŋ/.
Encoding
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.