I am assuming that the top item “tier” or type becomes available at level 92 (that is the title of the gear aka “Gothic Staff” or “Lich Wand” or whatever and so it stands to reason that the prerequisites to getting one of the ultra-rares to drop is to be level 92, it has to be the top tier item type, it has to have an item power of 820 (as previously noted).
Possibly a reason to grind to level 92 at least?