The ability can trigger only once each turn for each opponent
This wont be remiss for MtG veterans, but Ive found across other pre-con products this sort of information invaluable, moreso for newcomers and kitchen table players who arent as well versed in all the strategic angles of each color
Sending someones hard-earned graveyard back to their library can be a back-breaking play against reanimator decks, Underworld Breach, or even Thassas Oracle, as your opponent will often have too many cards in their library to win the game after putting their graveyard back into their library
Across a number of games at Gen Con, it was this change in focus from fighting to racing that became my first big realisation that Lorcana is far more than I cynically expected from a Disneyfied TCG

Cadira, Caller of the Small + Doubling Season + Trostani, Selesnya's Voice Infinite ETB, infinite lifegain, infinite lifegain triggers, infinite LTB, infinite sacrifice triggers, infinite death triggers, infinite creature tokens, infinite storm count Archfiend of Depravity + Archfiend of Despair Infinite sacrifice triggers for opponent's creatures over turns, Infinite amplified life loss for opponents over turns, Opponents lose the game Archfiend of Depravity + Archfiend of Despair (Denial Lock) Recurring board wipe for opponents, recurring life loss for opponents, severe board control, opponents cannot gain life Knight-Errant of Eos's Deep Search & Board Fill Put up to four creature cards from the top of your library into your hand, enabling a powerful board presence or assembling a non-infinite combo