You can read manga one piece chapter 691 at this online popular manga site. Here is the summary of One-Piece chapter 691: The chapter opens by showing the top half of the bisected building falling back down, presumably sealing the gap, although some gas begins to leak in. The giant children are scared at first, but Sanji and Nami convince them to keep running. The G5 marines soon spot Zoro approaching, carrying captain Tashigi while she demands he put her down. Upon seeing the marines she realizes he has gone wrong. Sanji is furious that Zoro picked her up, while Nami tries to make him turn back.
Meanwhile, Chopper's medical room starts to collapse due to Law's attack, and the G5 medics carry both him and Mocha away, while he continues to monitor her condition.
in the SAD room, Vergo has been completely dismembered by Law, and his body parts are attached to a railing. he calls Law's plan a messed up attempt at revenge, and taunts Law saying that he'll regret it, and cannot replace Doflamingo with power alone. He then asks Smoker to agree with him, but Smoker remains silent, while Law cuts Vergo's head in half one last time, and states that Vergo does not need to worry about Law but his own self since the room will soon be completely destroyed, before leaving.
Meanwhile, Caesar realizes his plan is falling apart, and becomes increasingly enraged. He orders his men in the secret room to open the shutters, letting Shinokuni flood the room, and when they argue that there are over a hundred men under Caesar's command left in the room that will be killed, he snaps at them, referring to them as Guinea pigs and scum, and that since nobody cares he can keep killing and replacing them as many times as he wants. His remaining minions are shocked into silence by this, to the extent that the ones previously fighting Usopp outright ignore him while he steps back to hide.
Caesar's puzzled reaction to their shock convinces some of his men that he is bluffing, and didn't actually intend to hurt them, and, believing it to be a complex strategy, open the vents as ordered. As the gas fills the room, Caesar somehow sucks it into himself, and begins to grow huge, before taking on a more demonic form. Gloating about his new found might, he promptly murders all of his men remaining in the room as a show of strength, enraging Luffy. He then continues that even peaceful countries have expressed an interest in his genocide weapons, and that he plans on supplying everyone, ultimately becoming the ruler of a war-torn "land of the dead". Luffy appears to flee, telling Momonosuke to protect Brownbeard.
In the secret room, Caesar's remaining forces are horrified by his casual murder of their allies, and realize he truly does not care about them. Ussop then interrupts, asking if he can use the control panel to help his friends and allies escape. Caesar's subordinates point out that Luffy has abandoned him, watching him run down the corridor on the monitors. Usopp counters that Luffy would find things easier if he did, but that was not in his nature. As he speaks, Luffy, still enraged, is seen to stop and turn, apparently assuming one of his gear second stances. As Ussop continues to talk about Luffy's positive characteristics, his captain continues by blowing air into both his arms, expanding both of them to activate gear third. He then begins to charge back down the corridor towards Caesar, both his arms stretching out behind him. Usopp states that Caesar is the type of person Luffy hates most, and that Luffy won't forgive him. The chapter ends as Luffy infuses haki into both of his stretched arms, and jumps towards Caesar, entering a pose similar to that of his Bazooka attack, while stating that he does not want to see Caesar's face any more.
