To celebrate the 40th anniversary of their creation, the LEGO minifigures are throwing themselves a birthday party, costumes strongly encouraged.  This series has a literally & figuratively odd number of entrants thanks to the presence of a bonus "chase" figure, of which only one is placed in every box of 60 individual blind bags.

Videos:
LEGO Series 18 Collectible Minifigures reviewed! All 17Full box worth of LEGO Series 18 minifigs! Rarity & distribution