gasp ... it's only like the #1 DApp on Steem ... OK, I made that up, but I'm sure it's up there :P
It's a Trading Card Game (a (VERY) short overview: you collect cards from which you select a 'team' which then battle against a team from someone else's collection).
Check out #SteemMonsters ... lots of people blogging about it.
Hehehe I thought that would shock you LOL! Giving my age away not so :):) Think it's about time I start looking, I always ignore those posts...