Add end mark at the end of every article

The end of the_content() is a </p> tag.
It sounds like you want to include your end mark before that last </p> tag.
If so, you need a function that filters the_content().

Something like:

function yang_end_mark( $content ) {

   $content = substr_replace($content, '<span>&#128282;</span></p>', strrpos($content, '</p>'), strlen('</p>'));

   return $content;

}
add_filter( 'the_content', 'yang_end_mark', 25 )

;