This mill deck is pretty solid, but there are some cards I don't really recommend, like Dimir Signet, Keening Stone, and Curse of the Bloody Tome (You can mill your opponent faster with Jace Erasures or Memory Erosion), simply because they don't help you mill as much cards as you may want. I recommend destroy cards like Go for the Throat and counters like Counterspell. I also recommend adding Vision of Beyond and Mystical Tutor (that is if you have the money for it).
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NO NO NO. No decimator web, no Circu dimir lobotomist, no haunting echos. They're time-inefficient and not worth playing. I have a mill deck myself, the highest summoning cost is 5, and the only cards that cost five in that deck are two traumatizes and an archive trap. Swap out Dimir signet, keening stone, bog witch, and Szadek, and dark ritual, and put in jace erasure, hedron crab, augry owl, preordain, doom blade, soul reap, etc. A mill deck is based primarily on very very cheap cards that can ignore the living hell out of your opponent.