Disclaimer: I obviously own nothing. This story is fanfiction.
One Last Explosion
When Dean learned that his best friend of 92 years had passed away in his sleep, he felt despair over the loss of his friend and bittersweet about how his friend had died. He was glad that Seamus hadn't had a particularly painful death, but he also knew that the Irishman would have preferred to leave life with a bang.
Ever since Seamus was a child, he unintentionally caused things to explode around him. Things no one would ever think could explode he caused to explode whether it was a feather or a glass of water. Over time, he came to also be able to make things explode intentionally. During the war with Lord Voldemort, Seamus's proclivity for pyrotechnics came in handy, allowing the Wooden Bridge to get blown up and preventing Death Eaters from coming across it to Hogwarts.
After the war, Seamus got work as a pyrotechnician, and it turned out to be his dream job. A few years later, he got married; Dean was his best man, and when Dean married two years later, Seamus was his. Over a period of several years, Seamus became a father of three children, two girls and one boy. One of his daughters, the youngest child, Grace, shared her father's penchant for unintentionally making harmless objects explode, much to everyone's amusement.
Seamus lived a long, happy life for the most part. It was only after his wife succumbed to a rare magical illness she'd contracted from one of the magical plants she'd recently discovered and was in the process of studying that he came to be less carefree. The children were already grown by that point, so Seamus had found himself alone in a house that was far too big for just himself. It didn't take long for Seamus to begin drinking too much and too frequently for anyone's comfort. Dean was the only one who managed to break through to him and get him the help he needed to process his grief more healthily.
Seamus never remarried, but he did come to find joy in life again. In his later years, he spent a lot of time with his family and particularly his grandchildren, who loved watching him blow up various items.
It was on an anniversary of his wife's death that he died.
Dean and his wife helped Seamus's children with the funeral arrangements. The children may as well have been their nieces and nephew with how close they were, and they knew how hard it was to lose someone close, especially the last of one's own parents. Dean and his wife had had to bury all of their parents by this point.
The funeral, which was held outside on a grassy hill starting at sunset, had a good-sized turnout. Besides all of the Finnigans and the Thomases being present, all of Seamus's friends and former coworkers who were still alive attended. There was no casket present.
Many came up to the podium at the front to speak to the audience about Seamus. Dean was the last to go. Seamus's children felt that he, as the person who had known and been closest to Seamus the longest, should speak last and present the final farewell that they had all agreed upon.
"...You've probably all been wondering where Seamus's casket is, and I have to tell you that there isn't one. Seamus, as you all know, loved explosions. If he had had a say so in how he died, he probably would've wanted to have gotten blown up. Alanna, Shaun, Grace, and I discussed it, and we all came to the same conclusion. Muggles have various ways that they handle people's bodies after they die. One unique way involves turning the person into fireworks. We felt it would only be fitting for Seamus to become fireworks and cause one last explosion." Dean turned towards Grace and gave her the signal to begin.
Grace set off the fireworks that her brother and sister had brought over while Dean gave his speech. The fireworks created brilliant patterns in vibrant colors, but most of all, they were loud and explosive and everything that their father would have wanted. In this way, Seamus Finnigan left the world with a bang.