I'm not a big lore person... But I've read around some rumours which sound somewhat convincing. Plus I don't play the Alliance hence I'm not familiar with the Anduin story-line in Legion too. :/
Is Anduin actually the son of Arthas Menethil (the ex-Lich King) ?
Can anyone provide some insight?
Sources:
https://www.reddit.com/r/warcraftlor...son_of_arthas/
Older: https://www.reddit.com/r/wow/comment...d_anduin_wrynn
and http://www.scrollsoflore.com/forums/...d.php?t=217236