Its the hottest thing on the Steem blockchain currently. Its a digital card game in which you can win free rewards while playing. For example I just got finished playing in a 50 SBD free to join tournament. A few days ago there was a 1000 SBD tournament. The tournaments are all manual at present but I have heard a rumor that auto tourneys are not too far behind.
If you are going to sign up don't forget to use my affiliate link so you qualify for some free rewards next time around.
https://steemmonsters.com?ref=rentmoney
Some of the cards found in packs have sold for upwards of $3000 in the past. You can collect / trade and sell for free but if you want to play the prize added tournaments or league you will need to pay $10 for the starter pack in which gives you 30 cards to start your collection. You can sign in using your Steem details.