The night of the eclipse begins with another bloody ritual, the sacrifice of maidens who perform a ceremonial dance. Quatermain is tricked into indicating the first victim when Twala asks him which of the girls he thinks is most beautiful. The first victim, Foulata, begs asylum from Captain Good, who assures her he will protect her. When Scragga attempts to sacrifice the girl anyway, Sir Henry steps in to protect her from the attack, killing Scragga in the process. The white men and Ignosi’s supporters are trapped in a face-off with Twala and his men when the predicted eclipse occurs, casting Twala’s people into panic and giving Ignosi, the white men, and their allies a chance to escape and regroup.
Once the men rendezvous, they await Infadoos’ arrival with news of who stands with them and what Twala intends. Civil war is imminent, and although the best of the Kukuanas throw in their lot with Ignosi, sheer numbers favor Twala. Twala divides his forces to attack Ignosi on three fronts; Ignosi’s men repulse the attack, but with heavy losses. Knowing they cannot withstand further assaults, Ignosi chooses to attack Twala directly, but with an eye to using the landscape strategically. The crescent-shaped plateau upon which Ignosi is encamped allows him to divide regiments to march around either arm of the crescent while a third force pushes down the center to hold the pass against Twala’s army. Twala falls for the gambit, committing his army to the narrow pass. The Greys—the bravest and most skilled of the Kukuanas—hold the pass until they are wiped out, giving the other two regiments time to encircle the plateau and catch Twala’s army in a pincer maneuver. Twala’s army is defeated; the survivors and their king hasten back to the safety of Loo to await the coming siege.
Ignosi offers the remnants of Twala’s army amnesty if they lay down their arms and open the gates of the city. They comply, leaving Twala to face Ignosi alone. Ignosi and his men arrest Twala and declare him a murderer and subject to execution. Twala calls upon the ancient laws of the Kukuanas to determine his means of execution: single combat with Sir Henry Curtis, the murderer of his son. Sir Henry agrees, and the two men fight. Sir Henry barely holds his own, but wins by decapitating Twala.
Her chosen ruler now gone, Gagool is captured and forced by Ignosi to lead the white men to the “Three Witches” mountains, wherein lie King Solomon’s Mines. Gagool leads the men to the Place of Death, where they witness the bizarre burial rites of the Kukuanas. Gagool opens the secret door to Solomon’s treasure chamber by a hidden trigger, allowing the men to enter a corridor leading to the treasure. Foulata, who has accompanied them, feels faint and cannot continue to the chamber. When the men reach the chamber, they are amazed at the riches of King Solomon’s diamond mines; as they stare at the precious diamonds, Gagool sneaks out behind them to seal the door. Foulata sees Gagool’s treachery and attempts to stop her, being mortally wounded in the process. Foulata’s efforts delay Gagool a split second too long—the old wise-woman is crushed beneath the stone door before she can make her way out.
Foulata dies, leaving Captain Good too stunned to assess the reality of their situation: they are buried alive. The men are overcome with despair, but suddenly realize that there is an air supply to the chamber and begin frantically looking for its source. Finding a stone trapdoor in the floor, the men pull the ancient access way open and escape down into the tunnels beneath. On the way out, Quatermain grabs a handful of diamonds and puts them in his coat pocket. They attempt to escape via an underground river, but the current is too strong and deadly; they go another direction and eventually find their way out through a hole dug by some wild animal. They have emerged amid several animal burrows and cannot find the way back into the mines by this route.
The men are welcomed back to Loo by Ignosi and his people. Ignosi is gratified at Gagool’s demise, but Captain Good is saddened by the loss of Foulata. After many days, the white men indicate that they wish to return to their own homeland. Ignosi first becomes angry at what he perceives to be a love of wealth over friendship, but his anger is soothed by Quatermain’s comparison to Ignosi himself wishing to return to his homeland. Ignosi declares the men heroes among the Kukuanas who will always be welcome, even as he enacts a policy of isolationism against any further white man incursions.
The men return home by a second, less hazardous route, and are surprised to find Sir Henry’s brother George Neville along the way. George had been injured and unable to continue his journey, so had settled into a hut near an oasis, unable to proceed or turn back due to his injury. With three strong men available, the party is able to get George home safely. Quatermain and Good decide to share the handful of diamonds with George for his pains in seeking King Solomon’s Mines. The men eventually return to Durban and part ways.
At the novel’s end, Allan Quatermain receives a letter from Sir Henry, indicating that the Dane has met and developed a favorable opinion of Quatermain’s son Henry. Sir Henry begs Quatermain to join him, Harry, and Captain Good in England, where an estate has opened up near Sir Henry’s own home. With the fabulous wealth available to him through the diamonds, Quatermain decides to join Sir Henry in a bachelor’s retirement to watch his son grow into his profession.