For X, one blue, and one black, target player puts the top X cards of his or her library into his/her graveyard. Plus, you gain X life. Of course, you could use this card to mill your own deck, as well. While it’s certainly too much of a mana sink for Constructed play, Commander players have found good uses for it. Szadek, Lord of Secrets and Mirko Vosk, Mind Drinker are two Commanders who often make use of this Sorcery. The most popular mill deck in Commander, Phenax, God of Deception, hasn’t really adopted it all that often, though.
Other Commanders that can take advantage of Psychic Drain include Wrexial, the Risen Deep, Dralnu, Lich Lord, Lazav, Dimir Mastermind, and Oona, Queen of the Fae. It’s a nice little uncommon that has often been overlooked as newer mill cards have been printed.