Warning! SPOILERS for Peaky Blinders season 6 ahead.
Polly Gray's (Helen McCrory) death. But Tommy has just lost his daughter and has been diagnosed with a terminal illness - this is his lowest point, and for the first time in the series, he accepts it.
In Peaky Blinders season 6 episode 4, Tommy s Arthur in the cellar, where Arthur is struggling with opium withdrawal. Before long, Tommy starts reminiscing about a time he was 9 and Arthur was 12: they were fighting over cigarettes and Tommy had Arthur on the ground. First, Tommy tells Arthur he thinks he "gave him one too many to the head" that time, and that is why Arthur has been "f*cked up" ever since (lifting the blame Arthur is feeling for doing drugs). Then, Tommy re another time when Arthur held off a whole group of policemen with a single boat hook - this made Tommy realize: "When I had you on the ground all them years ago, it’s because you let me win. You wanted me to win.”
Ever since Peaky Blinders' season 1, Arthur has shown frustration at his younger brother leading the Peaky Blinders. As of Peaky Blinders season 6, he is in no fit state to lead the family company, but neither is Tommy, and their discussion in the cellar proves exactly that. Season 6 episode 4 sees both Tommy and Arthur reach rock bottom, but it's the first time Tommy fully embraces his vulnerable state. He tells Arthur that after Polly's death he kept marching on, but after losing Ruby, he "stumbled and crashed." His ever-increasing ambitions seem to come to a halt, even though Tommy wants to "change the world" before he dies. With "just a few more yards left to go," Tommy finally gives Arthur what was his: the role of Shelby Company boss, or at least the head of the family. This proves Tommy has accepted his fate, which is certain death.
Tommy descending back into his drinking habits in the same scene further solidifies the idea that he has abandoned his ambitions and accepted his fate. He wants to complete his current deal and "get out". However, it seems unlikely it'll pan out as Tommy wishes: first of all, this is his most dangerous deal yet, involving fascists Oswald Mosley (Sam Claflin), Jack Nelson (James Frecheville), President Roosevelt, and Winston Churchill. Second of all and most importantly, Tommy has just been diagnosed with an inoperable brain tuberculoma, and his continued drinking and smoking show he is unwilling to keep his illness at bay.
Peaky Blinders' episode "Sapphire" sees Arthur ransack the Chinese opium den right after shaking hands with his brother. This goes to show that Arthur is far away from being fit to run Shelby Company (if anything, Ada is the perfect candidate for the role). But Tommy doesn't want Arthur to literally run the enterprise: he is just ceding his role to his older brother, something that was a long time coming. And since this is the same episode Tommy says he isn't a devil, "just an ordinary mortal man," it's safe to assume this was the moment Tommy accepted his tragic fate.
Peaky Blinders airs new episodes every Sunday on BBC.