Prop Sebastian's rugby journey has been unconventional, but at age 29 he is at the peak of his powers and hoping to help the Scots to a win that will keep their quarter-final hopes alive.
Born in Wales, Sebastian qualified for Scotland through his father and was as shocked as anyone to be starting on Saturday.
"I was quite surprised to be selected for the World Cup. It's a massive achievement to be selected in any squad so to get a starting slot is pretty surreal," he told reporters on Friday.
Sebastian played for Welsh side Scarlets before moving to Glasgow Warriors, where he was released after a year and decided to find a career away from professional rugby in 2016.
"That was a dark time," he said. "After I left Glasgow, when I had signed for them for a year, I went back home to have my first-born.
"I took a year out of rugby and didn't really end up having a job so applied to be a butcher and play semi-pro. I was there about two months but couldn't hack it any longer than that.
"I thought normal life is not for me, let's try to play rugby again. It made me open my eyes to what I could potentially do.
"Being a butcher, or any normal job, is quite tough. I’m not saying rugby isn't tough but the real world is scary."
He returned to Scarlets and got his career back on track, earning a first Scotland cap in 2021 and a move to Edinburgh ahead of the 2023-24 season.
"I've got four kids so I pushed for a place (in the Scotland team) just for them and my partner to try and make them proud.
"You have to take it week by week. Train as hard as you can each training session and hopefully that's enough by the time the weekend comes for selection," he said.