User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-162.205.216.169-20150709063502/@comment-3290441-20150711152145

Well, in each of the three volumes featuring him in some way that I've ready Rizevim's gotten a hold of some manner of extremely powerful artifact.

Holy Grail in 16 Agreas in 17 Apples of Life and Knowledge in 18

And we still have two volumes and at least one artifact left to go. Based on this my theory is: Use Holy Grail to revive apples. Consume apples to acquire knowledge to operate old Maou technology. Use old Maou super thing to take control over Trihexia to attack Great Red.