Strixhaven: School of Mages is a Magic: The Gathering expansion set. It is not part of a block. The set was released in April 2021.
Setting
The plane of Arcavios and its magical university Strixhaven were "created from scratch by the world-building team" for the 2021 Strixhaven: School of Mages post-block Magic set expansion. Dot Esports highlighted that Wizards of the Coast "made substantial efforts to make the colleges of Strixhaven feel distinct from their Ravnican counterparts. This was accomplished by leaning into the tension between each color pair, leading to conflicts between and within colleges. [...] Within each college are two separate philosophies that students can align with. [...] The team created a magic system that applies specific forms of spellcasting and philosophies to each college".
Colleges of Strixhaven
Strixhaven's colleges were founded by five Elder Dragons. Each college is defined by two color philosophies and "each college has two deans that are aligned with one color".
Lorehold: a college focused on history with "passionate scholars", "diligent researchers and daring adventurers". Witherbloom: a college for "goth bio majors" where students "draw power from the essence of living beings, whether that means enhancing nature or exploiting it" Prismari: a college for the theatre and art kids where students use magic to express themselves Quandrix: a college focused on math where students "study patterns, fractals, and symmetries to command power over the fundamental forces of nature" Silverquill: a college for "literature nerds" where students "wield the magic of words, from inspiring battle poetry to biting arcane insults".
Biblioplex
At the center of the university is the library known as the Biblioplex. This library contains both the Hall of Oracles and the Mystical Archive. This archive "is said to contain a copy of every spell ever created in the Multiverse"; this provides a lore explanation for the Mystical Archive cards where "all the uncommons are reprints of Standard-legal instants and sorceries, while the rares and mythics will continue to exist outside of Standard play. However, regardless of which rarity of spell you open in your draft booster, you’ll be able to draft and play it in Limited".
Planeswalkers
A number of Planeswalkers appear in the Strixhaven storyline such as Professor Onyx (better known as Liliana Vess), Kasmina (introduced in the War of the Spark), Lukka (introduced in Ikoria: Lair of Behemoths) and the twins Rowan and Will Kenrith (last seen in Eldraine).
Mechanics
There are draft archetypes themed around each of Strixhaven's colleges. Polygon reported that Magic is "running with this year’s emphasis on the Commander format — a multiplayer style of Magic that uses 100-card decks — each of Strixhaven’s colleges will have its own unique retail Commander decks".
This set added the Magecraft keyword ability (which provides a bonus to an instant or a sorcery) and the "Learn And Lesson" mechanic (which allows a player to cast spells from outside of the game or in the sideboard that they own). CBR reported that "the keywords and mechanics of the Strixhaven: School of Mages set revolve around the idea of magical students learning and growing over the course of a semester, but there is also the ward ability, which is more defensive in nature. [...] The idea is that these clever students are not yet masters, so they can't totally defend themselves, but they are smart enough to disorient their enemies and force them to decide how to spend their mana more carefully. [...] Players can attend class, and many spells in this set allow the player to learn lessons in the middle of a game".