Kill List

Jay and Gal are former soldiers who have become hitmen since they left the military. While Gal is laid back, Jay is still suffering from an unspecified disastrous mission in Kiev. Despite the urging of his wife Shel, he has not worked since, and they are running out of money. Shel organises a dinner party to which she invites Gal and his latest girlfriend, Fiona, a human resources manager. During the evening, Gal reveals he has a new job for them, which Shel encourages him to take. Meanwhile, Fiona goes to the toilet, carves a symbol on the back of the bathroom mirror, and takes a tissue that Jay had used to mop up his blood after a shaving accident. Jay accepts the job, and the two meet the shadowy client, who has a list of three people he wants killed. The employer unexpectedly cuts Jay's hand and his own, so that the contract is effectively signed in blood.
Their first target, a priest, appears to recognise Jay and thanks him just before being killed. The second name on the list is an archivist who keeps a collection of horrific, sickening videos of an undisclosed nature. He also thanks Jay, who, out of disgust for the videos, tortures and savagely beats him to death with a hammer. Jay insists on chasing down and killing the archivist's associates, and as Gal looks into their files, he finds a folder on himself and Jay, including details of their Kiev mission. Although they do not recognise it, the file includes the symbol that Fiona carved in Jay's mirror.
Gal informs Jay that while raiding the safe in the home of the second target, he took enough money to cover the total sum they would receive for the contract. The pair decide to abandon the contract and return home. When his cut hand becomes infected, Jay visits his doctor, only to find that his regular doctor has been replaced by another man who will only give him cryptic advice. Jay and Gal return to their client and offer to find replacements to kill the last name on the list. The client refuses and says that both hitmen and their families will be killed if they do not complete the contract. Shel takes their son Sam to the family's cottage for safekeeping while Jay and Gal go back to work.
Their final mark is a Member of Parliament who lives in a mansion. While observing the house, the pair witness a strange ceremony in the woods that culminates in human sacrifice. Jay opens fire with an assault rifle, and the leader of the ceremony presents himself for Jay to execute. The remaining masked cultists chase the hitmen into an underground complex, where Gal is disemboweled, forcing Jay to perform a mercy killing on his friend. Emerging from the tunnels, Jay flees to the family cottage and meets with Shel. When he goes outside, he sees that their car's tires have been slashed and lit torches have been placed around the nearby field. Jay attempts to locate their attackers, but he is knocked unconscious. Inside the cottage, Shel arms herself and shoots several invaders.
Jay awakens in the field, surrounded by the masked cultists, who strip him and place a mask over his face. He is confronted by his last victim, "The Hunchback", a masked and cloaked person armed with a knife. After a brutal knife fight, Jay triumphs, only to discover that the Hunchback was his wife with Sam strapped to her back. Shel appears to laugh as she dies. The cultists applaud and remove their masks, revealing Fiona, the hitmen's client, and the man from the doctor's office amongst their number. The film ends with Jay being crowned by the cultists.

Question: What does the client threaten if Jay and Gal don't complete their contract?