Back in 1999, Wizards of the Coast card games like Magic: The Gathering and Pokemon were rising fast in popularity. Seizing on their chance, Hasbro bought WotC that year for $325 million, giving the card company a huge boost in backing capital. Since then, Hasbro has had it's ups and downs, but has generally let Wizards its Magic brand do their own thing and stay on brand (extremely recent history notwithstanding). For every embezzling affair there is a planned Hasbro theme park complete with Magic: the Gathering rides, after all.
But, for nearly 20 years, Hasbro never went full "Hasbro Universe" by integrating their properties with each other or create big crossovers. Yes, it works for Marvel and poorly written network sitcoms, but Magic is a card game with no real TV, movie, or other entertainment backing behind it.
Until the year 2017, that is.
That year, Hasbro held HASCON, or HASbro CONvention, out in Rhode Island where the company's headquarters is located. With so many properties to introduce and show off (and Magic celebrating 25 years), the company looked for ways to play off each other. Amid the flurry of board games, children's toys, and 80s nostalgia was Magic: The Gathering. The company decided to have a few cards be tied into properties, and ultimately three were worked into promo cards and sold as a boxed set.
Dungeons and Dragons, a former TSR property that WotC acquired in 1997, got a card (which was then reprinted into the set Unstable). To borrow from golfing terminology, that one could be considered a gimme. Transformers got one too in the form of a dinosaur robot, but as that just sounds awesome, it also at least kinda makes sense.
And then for the third they chose...Nerf. Yes, that Nerf. And apparently they went full "Nerf or nothing" because they released this little gem as a promo card: Nerf War.
Unlike some other promo cards, the card was automatically illegal in pretty much every tournament due to it's card status, not being legal, oh, and requiring a Nerf blaster to actually play the card. Which, honestly, makes it a really good reason to get one.
The card's rules text makes this explicit: "Fire a Nerf blaster until empty at target library from at least two meters away. For each card knocked off that library, put it into its owner’s graveyard and Nerf War deals ½ damage to that player. (Foam darts only.)"
The Mark Heggen designed cards actually became something of a hit. In particular, Nerf War was liked enough to raise it's price considerably on the card market, with one going today for around $12.
(Related side note, Nerf once made a toy axe based upon the one the planeswalker Garruk wields as part of a San Diego Comic Con promotion.)
The success also led to more properties being made into special promo cards, including a My Little Pony themed set of three cards that came out in 2019 as part of a charity event. More were even set to come out in 2020 during a second HASCON until COVID-19 decided to just ruin everything.
With no second HASCON, and other non-media or game property having cards made of them, Nerf War remains an odd outlier of a card, even among the strange pantheon of promo cards. At least, that is, until Hasbro commands WotC to come up with a Furby card at some point.