As for the reaper thing...that's pretty obvious.

Scythe attacks
Voidsent oGCDS
Player+Voidsent is just that "symbiote" mode that I assume is a dps burst phase.

It reminds me of Berserker Guts, Hollow Ichigo, Eddie Brock/Venom and what have you.

I'm looking forward to learning more about both Sage and Reaper and what lies beyond what they've shown us.