The Reaper lands on Player A.

Player A rolls a 5 for The Reaper.

Player A chooses to "teleport" The Reaper to Player B who is on the Crown of Command.

Since The Reaper "teleported" and did not "cross the Portal of Power", is this legal?

KS

Death already exists in the inner region, it says in the rules. (Dicing with Death space.)
So death can't move into the inner region, period.