Bond Breaker

As great a group of friends as you are, if a situation is particularly dire for the group, and/or if something has a strongly negative effect for a particular character, that same group of True Companions might not be able to stick together anymore. If you want the story to become Darker and Edgier, have a main character drive the others apart. Have that character drive a wedge between the protagonists, dividing the Five-Man Band to a near-irreparable state.

This brings a great tragedy for a Five-Man Band, as well. For any group of True Companions, actually. Circumstances develop so badly that one character simply had enough, choosing a Face Heel Turn so devastating that the rest of the group won't be able to function from the shock. In other cases, The Resenter might take a few jabs at The Hero, on account of the former being Always Second Best. If they can't resolve the feud peacefully, friends of both are forced to take sides. Maybe the one that brought them together starts facing problems with their friends' personality flaws or psychological hang-ups; peer pressure kicks in, and The Hearts' attempts to resolve their friends' issues leads to more problems (maybe even more psychological distress on the part of The Heart) and broken bonds because of those problems.

Maybe the one that keeps them together starts hating his/her designated role as the Damsel in Distress or The Load, and their resentment leads to A House Divided, or they decide to leave the Five-Man Band, which falls apart because the one balancing them isn't there. In another scenario, The Hero Dies, and the rest of the characters fight over whether or not to revive him, despite the great risks involving such an act. In another scenario, the Implied Love Interest goes through so much Unresolved Sexual Tension, or Belligerent Sexual Tension, that they turn into the Yandere and take every step needed to keep the rest of their friends from influencing their love interest.

Whatever the case, good luck trying to bring the group back together after these depressing turn of events.

See also Breaking the Fellowship. Absolutely not related to an evil interrogator trying to break James Bond.

Anime & Manga

 * This is what happens in Naruto after Sasuke meets again his long lost and hated big brother Itachi, whom he swore to kill after the latter slaughtered every single member of their clan, save for Sasuke. The meeting can be resumed as No-Holds-Barred Beatdown meets Mind Rape. When he recovers (physically), Sasuke becomes obsessed with revenge while reporting his frustration upon Naruto (because of the latter progress). Meanwhile Sakura cannot do much but witness this. Sasuke leaves the Leaf not long after, to be taught by the current Big Bad. Bringing him back becomes his former True Companions' goal for the second part of the tale.
 * This was threatening to happen to the Straw Hat Pirates in One Piece when they reached the city of Water Seven. Robin disappeared for reasons unknown to any of her crewmates, and Usopp had a falling out with Luffy and left on his own accord. The rest of the time he was there, Luffy was trying his hardest not to let the rest of the crew disband. Averted in that he succeeded in keeping the crew together.

Film

 * In Bratz: The Movie, head girl Meredith is set upon driving the Bratz girls apart; after two years, she has succeeded in making them part of her pre-defined cliques, and they can barely speak to each other any more.
 * Such a case could be argued for Andrew Detmer in Chronicle, though the fault also lies on his father for Andrew's Start of Darkness.

Live Action TV

 * Spike in Buffy the Vampire Slayer calls his role in the Scooby Gang's decomposition the "Yoko factor". He didn't create the problems they were facing, but made them likelier to anger and alienate each other.

Video Games

 * In The Answer for Persona 3, after the protagonist dies, Yukari grows adamant about reviving him/her by going back in time. This was so risky that it would've prevented the Big Bad from getting sealed, risking the entire world once again to another battle that the team wouldn't have won in any other way than if the protagonist hadn't died. Thankfully, it doesn't come to that, and Yukari gets better, but only after a fight with the rest of the team (Mitsuru sided with her, though).
 * Bastila Shan gets tortured into serving the Big Bad, and she confronts you in the Ancient Temple on Lehon. If you choose to side with Bastila, Jolee, Juhani, Mission and Zaalbar get killed, while Carth runs away into the Lehon Temple. The droids and Canderous are the only ones that remain with you, while Bastila becomes your apprentice. Note that Bastila isn't the Bond Breaker; it's you.

Western Animation

 * An impressively realistic example comes from the animated Teen Titans show - fans know the episode as "How Long Is Forever". After Starfire goes through time while fighting the token villain of the week, she disappears from the rest of the Titans' lives. What was a few seconds for her, were twenty years for the rest of the team, and none of them handled her absence well, since she balanced them out.
 * Similar to the Teen Titans example, in the 2003 Teenage Mutant Ninja Turtles show, Donatello was warped into an alternate future where he had disappeared for 20 years. The Turtles were disbanded, which lead to the Shredder succeeding in world domination, and Michelangelo flat-out tells Don it's because he wasn't there to balance them out. Both this and the Teen Titans are also variations on It's a Wonderful Plot.