https://www.wowhead.com/news/an-iconic-scythe-transmog-is-more-accessible-than-ever-with-legion-remix-378212
https://www.wowhead.com/ptr/item=253304/cosmic-soulsliver

Cosmic Soulsliver works similarly to Bones of Mannoroth from Mists of Pandaria Remix, being awarded when you defeat Argus, the Unmaker in Antorus.