Revision 9b5edadc-bdb9-4ffc-a771-d80b7ba72f57 - Code Golf Stack Exchange

##XPath 3.0 (and XQuery 3.0), 84 bytes

 codepoints-to-string((0 to 25)!(subsequence(((1 to 26)!(65 to 90)),.*.+1,2*.+1),10))

Explanation:

`(1 to 26)!(65 to 90)` is the alphabet 26 times

`(0 to 25)!(subsequence(XX, start, len),10)` takes 26 subsequences of this, each followed by newline

`subsequence(X, .*.+1, 2*.+1)` takes successive subsequences with start position and length: (1, 1), (2, 3), (5, 5), (10, 9) etc.

`codepoints-to-string()` turns Unicode codepoints into characters