I found the solution
<?php
$costumepost = "12709";
// args
$args = array(
'numberposts' => -1,
'post_type' => 'post',
'cat' => array(18),
'meta_query' => array(
'relation' => 'AND',
array(
'key' => 'costumeid',
'value' => $costumepost,
'compare' => '='
),
)
);
// query
$the_query = new WP_Query( $args );
?>
<?php if( $the_query->have_posts() ): ?>
<?php while ( $the_query->have_posts() ) : $the_query->the_post(); ?>
<?php echo get_post_meta($post->ID, 'artist', true); ?><br>
<?php endwhile; ?>
<?php endif; ?>
<?php wp_reset_query();?>