This card is an example for the set of how to integrate Flanking into the set other than on creatures.
Also, note that, whilst historically Flanking has mostly been limited to chiefly Knight creatures, I feel this has had a negative impact on the mechanic. I see the mechanic as being used to show the flavour of either swift creatures that can literally outflank their opponents, attacking from the most advantageous position,OR for stealthy creatures that can stalk their opponents and attack with the element of surprise, catching their foes flatfooted.
Therefore, whilst Flanking will show up on some Knights, it will also show up on other creatures that flavourfully warrant it.

Unfortunately flanking is a bad choice to be granted by an instant. The reason is that the creature has to have flanking before blockers are declared. Combat tricks are usually used after blockers are declared, and many, many players would try to use this instant to kill a 1/1 that's already been declared as a blocker. In most cases this could be a sorcery. The only exception is its use to suddenly create a blocker that can block without taking the flanking penalty. But I think the misplay risk is far too high, and this should be made a sorcery (or cut entirely).