add_rewrite_rule.php

<?php
function custom_rewrite_basic() {
	add_rewrite_endpoint( 'custom_slug', EP_PAGES );
	add_rewrite_rule( 'custom_slug/([^/]+)/?$', 'index.php?custom_slug=$matches[1]', 'top' );
}
add_action( 'init', 'custom_rewrite_basic' );

function custom_register_query_var( $vars ) {
	$vars[] = 'custom_slug';

	return $vars;
}
add_filter( 'query_vars', 'custom_register_query_var' );

add_action( 'template_include', function( $template ) {
    if ( get_query_var( 'custom_slug' ) == false || get_query_var( 'custom_slug' ) == '' ) {
        return $template;
    }

    return get_template_directory() . '/custom_slug-template.php';
} );