Diagonal Alphabet To the Input
Charcoal, 9 bytes

↘✂β⁰Ｎ↓✂βη


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How it works

↘✂β⁰Ｎ↓✂βη
 ✂β⁰Ｎ         the alphabet from position 0 to the input
↘               print diagonally, down and to the right
        ✂βη    the alphabet starting from the position of the input
       ↓        print downwards


This solution no longer works in the current version of Charcoal (most likely due to a bug fix), but the issue is resolved for 10 bytes with ↘✂β⁰Ｎ↓✂βＩθ.