Daniel 1:18 (ESV)

18 At the end of the time, when the king had commanded that they should be brought in, the chief of the eunuchs brought them in before Nebuchadnezzar.

The period of time set by Nebuchadnezzar for their training comes to an end and these young exiles appear before him to review their learning and to see whether they are to be admitted to further service and studies. It is Ashpenaz who brings all the boys who have received the training to appear now before Nebuchadnezzar.