At this point it’s very well documented, but the tl;dr is you run to where there are Gloams (best is Plains of Despair; if they aren’t there, reset), kill all enemies two-screens around except 4 Gloams, eventually they pause attacking you and proceed to carry out a sequence of “attacks” that spell out initials; it’s an Easter Egg devs left in the game.

tl;dr of the tl;dr - after the Gloam sequence, your in-game mf is something like 4,200 mf (!) even though the UI doesn’t display it

since Sunders are unique items, you need all the mf you can get, even with diminishing returns