Those pesky Devilkins are a type of challenge that appear on some levels. They hide behind un-daubed numbers on a Slingo® card and will sometimes change nearby numbers to prevent them from being daubed. A number blocked in this way will briefly have a red outline, then change to another number.
Devilkins will move around the card throughout the level. Daubing the number a Devilkin hides behind will cause it to move to another undaubed number. Creating a Slingo® using the tile a Devilkin is hiding behind will remove the Devilkin from the card.
You can use a Devilkin Douser Power Up to try to remove a Devilkin from a card.