While I am not that familiar, I have checked on Wikipedia and the site describes that “Heroes of the Realm is a custom made series of black-bordered celebration cards, recognizable by a special “Heroes of the Realm” card back. These cards are specifically created for Wizards of the Coast in very limited numbers, and handed out by the execs to some of their employees as a token of appreciation.” They are, however, not legal to play in any format given that the back of the card is not that of the regular Magic card. This makes them a unique and one-of-a-kind print promo cards.

This special edition started in 2017 with Chandra, Gremlin Wrangler, Dungeon Master for 2016 Dungeons and Dragons Reward, and Nira, Hellkite Duelist as 2016 Duel Masters Reward.

For 2019, we have the latest reveal: Sol, Advocate Eternal.