Tribute 1 face-up monster; Special Summon from your Deck, 1 monster with the same original Type and Attribute, that the Tributed monster had on the field but 1 Level lower. You can only activate 1 "Downbeat" per turn.
You can use Swamp Mirrorer to select any Type of Attribute and it becomes those properties, then you can use Downbeat to bring out any monster that is 1 level lower than Swamp Mirrorer but has the same Type and Attribute that you declared. You can use Downbeat on Meitiel, Sage of the Sky, Zeradias, Herald of Heaven, Harvest Angel of Wisdom, or Bountiful Artemis, in order to Special Summon The Agent of Creation - Venus from your Deck. The Agent of Creation - Venus is able to subsequently Special Summon Mystical Shine Ball from your Hand or Deck by paying 500 Life Points.
I'm not sure i get downbeat, for some reason I can't activate it if there's a lvl 3 monster on field even though I have many monsters in deck with same attribute and type. (all water and sea serpent/fish deck)
Makes perfect sense if you don't have any copies of Atlantean Pikeman, Imairuka, White Dolphin, Piranha Army or Fishborg Planter in your Deck, since they're the only Level 2 Fish and Sea Serpent-Type monsters in Duel Links.