# sed -n 'H;$x;$s,\n,,gp' file
       ^  ^  ^  ^
       |  |  |  |
       |  |  |  |
       |  |  |  For the last line only, globally strip all line endings in the pattern space and print the result
       |  |  |
       |  |  For the last line only, swap the hold and pattern spaces
       |  |
       |  Append each line to the hold space
       |
       Don't print any lines - by default all lines are printed following processing