Released in April 2011, The Promise is the fourth and final entry in Big Finish's second season / series of their Highlander Audio Series.
The Promise opens with Methos, who has been an occasional commenter in the previous three episodes, engaging in a conversation with the ancient immortal Dilijan.
Methos confirmed that he had no memory of his First Death and expressed scorn for the watercooler bonding experience most immortals enjoyed via discussion of their first deaths. He also expressed contempt for The Prize, he had no interest in earning it as soon as he learned of it.
Frustrated, resisting The Game and the killing it required, but driven by the urge to survive he resolved to research immortals in earnest. He met a historian named Simon and enlisted him to assist in his research. Simon discovered several ancient revelatory documents and invited Methos to view them. As the two were making their way to view the scrolls a band of raiders sacked the town, killed Methos, and burned the scrolls. Simon was horrifically burned and succumbed to his injuries.
After Simon's death Methos' hunted the raiders. He dubbed himself Death and killed the raiders, their families, their friends, people that owed them money, anyone who had been to the same taverns, everyone and everything that had ever had contact with them he killed and destroyed. When he finished wiping out the memory of the raiders he couldn't stop killing, he moved on to killing criminals, then people he didn't like, and eventually the why and who of his kills didn't matter, only the killing itself.
Dilijan pointed out that for a time when Methos rode with Dilijan's original band he had managed to control his murderous urges. Methos agreed and explained that even years later the urge was overwhelming, once he joined with Kronos there was no going back. He admitted that eventually he did grow tired and sickened by the killing.
Dilijan revealed that there were two events in his life he would never forget, the day Kronos turned on him and enlisted Methos in the slaughter of Dilijan's band and the next time Kronos and the Horsemen found him and wiped out his new mortal tribe. He carefully noted that Methos could have prevented both events if he had wanted to. After the second slaughter Dilijan went to ground and managed to avoid Kronos' attempts to find him.
With the Horsemen off his trail Dilijan found himself unable to ignore them or move on with his life. He admitted he was partially responsible for providing Kronos with the tools he needed to create the Horsemen. Dilijan encountered an immortal who sought Dilijan's skills as the legendary Pain Eater to ease his emotional burden. Dilijan trepanned the immortal and drained his Quickening, he replaced his Quickening with one engineered to compel the immortal to seek out and challenge Methos then lose. The immortal, Dorn, carried a Quickening engineered to force Methos to face his deeds and the pain they had caused, an excruciating experience designed both to punish and redeem Methos.
Dilijan explained in broad strokes how he manipulated and stored Quickenings, when Methos questioned his methods closer Dilijan revealed that the six immortals in the lab where he and Methos were talking were blank slates holding a thousand times more energy than a normal immortal. Dilijan literally wiped their minds and souls rendering them into living Quickening storage units.
Methos was horrified by Dilijan's 'batteries' declaring his actions worse than anything the Horsemen did. He questioned Dilijan's funding and equipment, Dilijan explained h was being funded by Tasker Industries although they were unaware of the full extent of his work.
Dilijan admitted that he had considered filling Dorn with despair to drive Methos mad but decided instead to use the opportunity to turn Methos from his path. The gesture was not altruistic, Dilijan intended to redeem Methos, even see him find happiness again, before he would instigate his final act of vengeance.
Dilijan warned Methos that without Dilijan's interference Methos would never have been able to stop killing, he would have continued until the present day and if Methos started killing again Dilijan wouldn't be able to stop him. Dilijan then took credit for the ship wreck in Brothers that divided the Horsemen. As Methos listened Dilijan admitted he carefully waited until Methos was happy, before acting against him again.
Methos described his lover Violetta, he said she was his one true love and when he first saw her it was almost the same feeling as being near an immortal. She was an opera singer and together they traveled the world for her work, they didn't spend a day apart from the moment they met.
Methos described his self-loathing and how Violetta's love for him encouraged him to be a better man and person. Emboldened by her love and the new confidence her love gave him he confessed a version of his past to her. To his surprise she took the confession in stride. She admitted she wasn't happy about it and couldn't forgive him as it wasn't her place to do so but she loved the man he was then. When he apologized for not telling her sooner she pointed out that it wasn't precisely the sort of thing to admit on a first date and she had suspected he was holding something back, he carried a sadness in his eyes and even his laugh that had tipped her off. She told him that he was clearly holding back more but she didn't care and didn't want to know, she loved the man he was now.
They spent another six months traveling and then returned to London to see the premiere of La Traviata. Prior to the opening Violetta purchased ice cream for them. As they were eating it Methos noticed it tasted odd, Violetta explained that she was given the ice cream for free from a strange man who told her the ice cream would, 'help us to realize our full potential' - a catchphrase common to Dilijan's catspaws. Methos slapped the ice cream away and taking Violetta with him attempted to find the ice cream vendor. However Violette grew weak, poisoned by the ice cream, and Methos could not find Dilijan or his acolyte.
Methos described Violetta's final days in detail, Violetta lingered for weeks and no doctor was able to ease her pain or improve her condition. Before she died Violetta made it perfectly clear that Methos wasn't to blame himself for her death then made him promise not to kill Dilijan. She knew that if he started killing he would become the killer he had once been and the man she loved would cease to exist. Methos reluctantly promised and Violetta sang him an aria from La Traviata before dying later that night. Since then he tells people that he considers opera boring because he can't stand to hear it and remember his loss.
His story finished Methos turned his attention back to Dilijan. He explained that he understood why Dilijan did what he did, even understood that Violetta was just a means to an end to Dilijan, but he cannot forgive her death. Dilijan demands to know if Methos will continue the cycle of vengeance and points out that one could say the two were now even.
Methos admitted that was possible but then countered that he did not tell Kronos to attack Dilijan but Dilijan chose to go after Methos. Methos explains that he waited over a hundred years to come after Dilijan because he would have pulled his head from his shoulders with his bare hands if he had come any sooner and broken his promise.
Dilijan asks how Methos found him, Methos explains that he realized Kyne and Dorn, the two immortals Dilijan had deliberately insterted in to Methos' life had somehow been manipulated by Dilijan via their Quickenings. Methos reveals that he had noticed, even back when he rode with Dilijan and observed Dilijan's experiments draining despair from other immortals, that Dilijan enjoyed the despair of others a little too much; he suspected Dilijan was addicted to despair.
When Dilijan protested that he had acted righteously when he sent Dorn to Methos, Methos countered that the reason he had to stop Dilijan was because he was convinced of his own righteousness, though he admitted that if Dilijan had stopped pursuing Methos after sending Dorn then he may have been in the right. But Dilijan didn't leave Methos alone after Dorn, he kept pushing Methos.
The two debated the issue until Methos tabled it as a useless argument and set up explosives to kill the six immortals Dilijan had been using as 'batteries'. When Dilijan suggested Methos leave him behind to die with them Methos reminded Dilijan that he had promised not to kill Dilijan. Instead, he will reverse Dilijan's process and send the energy from the six immortals back into Dilijan, in effect, filling Dilijan with the despair of six thousand immortals, or as Methos puts it, a thousand dark Quickenings at once.
Dilijan protests that his mind will be burned away, that Methos will commit an act that is far far worse than anything Dilijan or Methos had done until then. Methos countered that Dilijan had pushed him, a mistake Dilijan of all people should have realized. When Dilijan accused Methos of having started the cycle of revenge in the first place, Methos told Dilijan that he was now ending it, he pointed out that in addition to burning out Dilijan's mind he would be curing Dilijan's addiction to despair. In other words helping him to realize his full potential.
After the procedure Dilijan was a sniveling wreck. Methos sedated him to silence his whimpering then blew up Dilijan's lab and drove him to a mediocre mental institution of some description where he admitted him. Before leaving the location of the lab Methos has a quick conversation with a version of Violetta that apparently remains in his head as a sort of delusion / companion. 'Violetta' chastises Methos for skirting the Promise by burning out Dilijna's brain.
At some point later Methos visited the institution and observed Dilijan, an attendant noted that Dilijan was having a good day which meant no screaming although the medications were beginning to cease working. Methos asked for privacy telling the attendant he didn't want anyone to see him cry.
Methos explained to Dilijan that he can't remember anything or make new memories that last for any useful length of time because his mind is too full of agony. Dilijan asked why Methos came to see him suffer Methos replied: Because you got into a fight that you couldn't possibly win, and I don't care if the punishment outweighs the crime by a hundred to one, I did it anyway. Now, seeing you in agony makes me happy. Killing used to fill that need but I've kicked the habit. Now I can just come here and take pleasure in your pain. I suppose you could say you're my murder methadone.
Dilijan asked Methos how he can die and stay dead, he muttered that he thought he cut his wrists once and died but he didn't stay dead. Methos told Dilijan that he woudn't tell Dilijan how he could die, as Methos was leaving he told Dilijan he would come by to visit every hundred years or so then added, Maybe, next time, if you've been good I'll tell you what your name is.
As the door closed and Methos' steps faded Dilijan begged Methos to return and the voices of Kronos, Caspian, and Silas rose promising to be with Dilijan forever.