成れの果て is a poetic expression for the final, often tragic or degraded state that someone or something has ended up in. It carries a stronger and darker meaning than simply "the result" or "the end."
In the song, 快楽選んだ成れの果て describes the ruined world created by choosing pleasure above all else. This memorable phrase adds a dramatic, literary image of consequences and downfall.
"In The Story" plays like the prologue to a dark fantasy game. It opens in a world with no morning, "朝のない闇の世界" (a world of darkness with no morning), where people have been robbed of "自由と眠り" (freedom and sleep), and every dream gets drowned out by "不協和音" (dissonance, literally "discordant sound"). Music is the thread through the whole thing: a frozen melody, then "破滅のメロディー" (the melody of ruin), and at the very end a call to play "五つのメロディー" (five melodies).
The second verse gets surprisingly direct about the planet. "化学に呪われた風" (wind cursed by chemicals) and "異臭放つ腐る海" (a rotting sea giving off a foul smell) describe a wrecked earth, the price of having "快楽選んだ" (chosen pleasure). Then the singer turns to you: "今君の住む世界も" (the world you live in now, too). The chorus is the hopeful part. History keeps coming round, "めぐりめぐる歴史", and he hands it "そっと君だけに" (quietly, to you alone), because "誰も皆 小さな天才" (everyone is a little genius).
Useful for learners: "君にだけにしかできない" (something only you can do) shows the "しか + negative" pattern, which means "only" even though the verb is negative. The song ends on a cliffhanger, with you standing on a hill "歩き出そうとしている" (just about to start walking).