At the time of decimalisation in 1971, the United Kingdom Crown coin (equal to 5 shillings) was redenominated as a 25p coin and pieces struck prior to 1990 - all the way back to 1818 - continue to be legal tender for that amount. The old denomination had been a regular circulating coin in the 19th century, but in the 20th century it became more of a commemorative denomination.
The Royal Mint continued issuing commemorative coins in the same format - using the traditional coin dimensions, but in copper-nickel. These were initially also denominated as 25 pence but the "Crown" was changed in 1990 to a commemorative £5 (five pound) coin, having the same dimensions and weight but with a face value twenty times as great.
The denomination is used to mark special occasions, usually royal in theme, rather than for use in general circulation. Although not circulating, these coins are legal tender.
This is the first design in the second part of the Myths and Legends range of coins by the Royal Mint, and the first in the range to be issued in a non-bullion format; it features legendary King Arthur.
The mint says about it:
The first commemorative coin in our Myths and Legends Collection honours King Arthur, the fabled leader of the Knights of the Round Table.
King Arthur is a mythical figure often depicted as a fearless warrior, noble king and the chivalrous leader of the Knights of the Round Table. A legend recast and retold throughout the ages, Arthur’s story first arose in the Welsh chronicles of the ninth and tenth centuries and the legend’s ambivalence continues to perpetuate its appeal.
The illustrator David Lawrence has skilfully crafted a design featuring the legendary king at its epicentre, garbed in regal apparel as he brandishes Excalibur, his legendary sword. Camelot, the fabled castle where the Knights of the Round Table held court, is discernible in the background. | 
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