This set introduces new cards into Modern without them ever being legal in Standard. It contains new cards and only reprints which are currently not legal in Modern. For this, it was rigorously playtested by Play Design. In addition there are five basic lands, and a Buy-a-Box card which is also a new-to-Modern reprint. The powerful new card options, some with old mechanics, are mixed with flavorful updates for favorite characters.
Modern Horizons was originally dubbed as “Timespiral / Planar Chaos 2” in concept due to set complexity. Mark Rosewater has called it a a “love letter” to the Time Spiral block. It won’t be an entry level set and has more than ten mechanics.