Complex sentence: That evening, Snow announces Panem's third Quarter Quell, a version of the Hunger Games every 25 years that contains a special provision: all tributes are selected from the existing pool of victors.
Split into simpler sentences: That evening, Snow makes the announcement of Panem's third Quarter Quell, a special version of the Hunger Games which happens every 25 years and contains a special catch. This year, all tributes are to be selected from the existing pool of victors.