In one of the final episodes, Chez notices Isaac is immortal when he's trying to save him from falling off the train. Isaac also got shot, and beaten down.
However, he didn't drink the "sake" he and Miria stole from the mob, did he?
How did he and Miria became immortals?
Same way as Gandors and Martellos.
Box with elixir of immortality went from what's-his-name brewer to Dallas Genoard, who then left it at Gandors' place. On Scilard's orders, Dallas recovered the box, but then he was robbed by Isaac & Miria, who took the box with them to Firo's inauguration party, where all present drank the elixir.
it differs very slightly between the anime and novels, but the answer is, they drank it during Firo's promotion party. (though when and how the elixir got there is the main difference between the versions).
In the anime, there's 1 party, where as in the novels, there's 2. (the night Szilard shows up is basically an impromptu after-party after Issac and Miria steal the box from Dallas and finding out it's liquor and decide give it to the Martillos for treating them the night before during Firo's promotion (which is where the real elixir was drunk)).
In the novel Firo was actually the one who stole the Elixir, and the box everyone was chasing around all book long was just normal alcohol Firo had traded out with the elixir while Barnes was unconscious.
Novel Firo is kind of a jerk, and basically decided to "test" Barnes. If he was nice and respectful for being saved, Firo would give the liquor back, if he was a dismissive jerk, Firo would keep it. Now it does make some sense, as the Begger/Mugger who attacked Firo actually happened that morning (Novel Firo beats the tar out of him without a scratch) and he was fuming over someone attacking him after he'd tried to do a good deed for them, and decided to test Barnes because of it, see if he was also an ungrateful asshole.
Since he thought the stuff Barnes had and was so protective of was some extremely high dollar imported liquor, where as the stuff he was originally bringing to the party was a bit on the cheap-side.
