J.R.R. Tolkien had four children. In 1920, his three-year-old son John had some questions about Father Christmas. That's when Tolkien became Father Christmas, as he wrote the charming letter you see above to his son. He drew the picture, too.
For the next 23 years, every Christmas Eve, Tolkien wrote a letter to his four children from Father Christmas. What began as short, informative letters—“I am just now off to Oxford with a bundle of toys”—evolved into longer tales about life at the North Pole. The 1932 letter begins, “Dear Children, There is alot to tell you. First of all a Merry Christmas! But there have been lots of adventures you will want to hear about. It all began with the funny noises underground … ”
What follows is a tale of a beloved polar bear, mysterious caves filled with goblins, and their heroic counterparts, the Red Gnomes. As with The Hobbit, which Tolkien wrote and published in this period, the letters contain entire worlds, with invented languages and histories, alongside detailed illustrations.
You can see -and read- a few of those Father Christmas letters at Atlas Obscura.