3 added 9 characters in body
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CJam, 1212 8 bytes

4 bytes saved thanks to Dennis!

l_$q\@er

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Explanation

l        e# read a line
_$       e# fancy way to obtain string "abc...z". Thanks to Dennis!
q        e# read the rest of input as a string with newlines
\@       e# swap, rotate
er       e# transliterate. Implicit display

CJam, 12 bytes

4 bytes saved thanks to Dennis!

l_$q\@er

Try it online!

Explanation

l        e# read a line
_$       e# fancy way to obtain string "abc...z". Thanks to Dennis!
q        e# read the rest of input as a string with newlines
\@       e# swap, rotate
er       e# transliterate. Implicit display

CJam, 12 8 bytes

4 bytes saved thanks to Dennis!

l_$q\@er

Try it online!

Explanation

l        e# read a line
_$       e# fancy way to obtain string "abc...z". Thanks to Dennis!
q        e# read the rest of input as a string with newlines
\@       e# swap, rotate
er       e# transliterate. Implicit display
2 added 73 characters in body
source | link

CJam, 12 bytes

4 bytes saved thanks to Dennis!

lq'z,'a,-@erl_$q\@er

Try it online!Try it online!

Explanation

l        e# read a line
_$       e# fancy way to obtain string "abc...z". Thanks to Dennis!
q        e# read the rest of input as a string with newlines
'z,'a,-  e# string "abc...z"
@ \@       e# swap, rotate
er       e# transliterate. Implicit display

CJam, 12 bytes

lq'z,'a,-@er

Try it online!

Explanation

l        e# read a line
q        e# read the rest of input as a string with newlines
'z,'a,-  e# string "abc...z"
@        e# rotate
er       e# transliterate. Implicit display

CJam, 12 bytes

4 bytes saved thanks to Dennis!

l_$q\@er

Try it online!

Explanation

l        e# read a line
_$       e# fancy way to obtain string "abc...z". Thanks to Dennis!
q        e# read the rest of input as a string with newlines
\@       e# swap, rotate
er       e# transliterate. Implicit display
1
source | link

CJam, 12 bytes

lq'z,'a,-@er

Try it online!

Explanation

l        e# read a line
q        e# read the rest of input as a string with newlines
'z,'a,-  e# string "abc...z"
@        e# rotate
er       e# transliterate. Implicit display