Responses
that promise is not a mystery..really.!!
Meredith offered to release his master and the Helios students, in return, Rood has to stay with him..then he pierced Rood's stomach.. remember now??
http://www.mangago.me/read-manga/black_haze/bt/c144/pg-6/
What kind of promise was made between the Demon Blow and Meredith? A new mystery has to be solve again.
And it looks like.. on the chapter 0, there's a skeleton woman. She told the young Rood (in the tower, right after he merges with the Demon King, before Kiel found him) that the powers of the mages was given by the Demon King. Maybe this is the result of that promise?