The most basic prerequisite is having a card with the level and colour required by the card on the field while also paying a Memory cost

EXTERNAL LINKS - ALL FIXED Ask Wizards December 2003 Broken Link Ask Wizards - December, 2003, Feature Article - December 1, 2003 - Wayback Machine Land of the Rising Fun - Feb 14, 2005 - Wayback Machine 1996 World Champion, Magic Arcana Broken Link 1996 World Champion, Magic Arcana - May 9, 2002 - Wayback Machine World Records, Magic Arcana Broken Link World Records, Magic Arcana - September 14, 2011 - Wayback Machine World Quiz, Magic Arcana Broken Link World Quiz, Magic Arcana - December 1, 2014 - Wayback Machine Rarest trading card 1996 World Champion Original Art Video INTERNAL LINKS Hobby Japan Life Counters 1996 World Champion - Unique Cards SET NUM CARDNAME ARTIST SOURCE DATE INFO 001 1996 World Champion Christopher Rush Winner card of the 1996 World Championship 1996-08-14 YES Shichifukujin Dragon 1996 - Unique Cards SET NUM CARDNAME ARTIST SOURCE DATE INFO 002 Shichifukujin Dragon Christopher Rush Celebration card of the Japan DCI Tournament Center 1996-?-

(1) If Visas Starfrost is on the field: Target 1 Effect Monster on the field
It's community shorthand that distinguishes this Supernova-combo archetype from other Law leaders in the card pool
Very unlikely
It was first released in-game on September 25, 2018