What are the Ultimate Scream Card in FIFA 19

Halloween is coming! Many sites offer the Ultimate Scarem Cards, I can't understand the feature of that cards. So, who can explain the features of this card for me? Thank you!

The FAQ here states:

"Ultimate Scream is back in FIFA 19 Ultimate Team, and a squad of 21 shapeshifting beasts has been let loose in FUT. These beasts have been released in the form of special player items that will have a boost throughout the FUT season to two of their six stat categories. These Ultimate Scream players are unlike any other in FIFA 19 Ultimate Team; not only will each player have two of their six attributes boosted at all times, but the stats that get boosted will change throughout the season.

When these boosts will occur, how often they occur, and which two of the six attributes are shifted each time is a complete mystery."

I suggest you read through the rest of the FAQs in the link above for more information.