"Chapter 18" is the tenth episode of the second season of the American cable television series Legion, based on the Marvel Comics character David Haller, and connected to the X-Men film series. It is the 18th overall episode of the series and was written by series creator Noah Hawley and co-executive producer Nathaniel Halpern and directed by Dana Gonzales. It originally aired on FX on June 5, 2018.

The series follows David Haller, a "mutant" diagnosed with schizophrenia at a young age. Struggling to differentiate reality from fantasy, Haller tries to control his mutant powers and the sinister forces trying to control them while evading a government agency, Division 3. Summerland and Division 3 eventually team up in order to catch Amahl Farouk from finding his original body. In the episode, the team arrives at the desert, with everyone having different goals, but David sets out to find and kill Farouk.

According to Nielsen Media Research, the episode was seen by an estimated 0.467 million household viewers and gained a 0.2 ratings share among adults aged 18–49. The episode received generally positive reviews from critics, who praised the performances and character development, although some viewed the episode as a needless two-parter season finale while the pacing was also criticized.

At the desert, Syd (Rachel Keller) exits her tent, discovering a giant plughole in the desert ground. Suddenly, she is hit with a rope and dragged down the hole, while David (Dan Stevens) wonders where she went.

Syd reaches the bottom of the hole, ending in a room with Melanie (Jean Smart). Melanie attempts to convince Syd that she should not trust David, indicating that he may not love her. In the desert, David reaches the Mi-Go monastery, where he finds Farouk in the body of Oliver (Jemaine Clement). Using his powers, he starts to torture Oliver so he can reveal Syd's location, which he refuses to disclose. All of this is watched by Syd and Melanie, who wants to show him that David is a monster.

With Syd still believing in David's intentions, Melanie then attempts to showcase how David constantly met with her future self. She also shows her that David is the reason for the incoming apocalypse. Unaware to Syd, Melanie is possessed by Farouk (Navid Negahban), and sends the minotaur to kill David. In the desert, Cary (Bill Irwin) and Kerry (Amber Midthunder) arrive with Division 3 operatives at the plughole. From inside the plughole, monks emerge and emit a high-pitched frequency that fends them off. Kerry overcomes the frequency and subdues the monks, until more monks emerge from the plughole.

With Farouk leaving Oliver's body, he finally gives up on the torture and reveals that Syd was with Farouk. Clark (Hamish Linklater), the Vermillion and more operatives arrive at the monastery with a giant tuning fork named "The Choke". However, Farouk has located his body and abandons Melanie's, finally accomplishing his goal. Back in the plughole, a monk takes Cary inside and Kerry decides to go after him while Lenny (Aubrey Plaza) stays above, having retrieved the weapon: a paintball gun. As the Vermillion fight the minotaur, Cary escapes. Kerry encounters the minotaur, and fights it with Syd's help.

Cary discovers David in a throne room, having recognized to torturing Oliver and acknowledging that Farouk may have found his body already. David's behavior seems to have changed, who now seems intent on killing Farouk. Back in the monastery, Clark and the Vermillion try to activate the Choke, which can dampen mutant abilities. Farouk arrives, knocking out Clark and causing the Vermillion to retreat. He uses his powers to throw away the Choke and marvel at his full power. David suddenly arrives, reciting The World's Angriest Boy in the World.