https://mtgarena.pro/decks/ub-mill-21/

"Not the safest deck in an Aggro heavy meta, but I just love mill decks so much.

Psychic Corrosion Patient Rebuilding and Drowned Secrets do most of the mill work with Millstone thrown in because I didn't have 2 more secrets. Discard in Thought Erasure and Disinformation Campaign should help you remove big threats before they hit the board, with Price of Fame for backup. Unexplained Disappearance is there to bounce back pesky creatures with counters. Nightveil Predator is mostly there as an annoying distraction that is hard to deal with and can remove some creatures itself. Etrata, the Silencer and Thief of Sanity function mostly the same way as threats that have to be dealt with, with the former being an alternate win condition if you get lucky (or unlucky).

Obviously mill decks do work for popular Golgari and Izzet decks, so the sideboard is built around dealing with those. Otherwise you can hope to use discard to get threats like Crackling Drake or Enigma Drake out of the way or slot in some more removal. Ritual of Soot and Sleep are there to deal with aggro decks like Merfolk

Unmoored Ego is so good it should probably go in the main deck"