A player only casts spells, not lands
Activated ability costs appear before a colon (:) in a cards rules text, or, in the case of some keywords, before a colon in reminder text
OP12-116 We'll Ring the Bell Waiting for You
Thats good, but it also reads, Whenever you cast a spell, each opponent reveals cards from the top of their library until they reveal a land card, then puts those cards into their graveyard. Being able to cast any spell and then mill everyone is absurd, and the fact that this can then also kill people if needs be is unparalleled
Beyond that, theres a good amount of curve filler with relevant text in cards like Dusk Legion Sergeant, Elendas Hierophant, and Redemption Choir