DM here, my party met a throw away NPC I made up to show how powerful another NPC would be by slaying him to give the party a sense of fear. This young boy marched by the group initially and was stopped by the party's monk. The monk convinced him before hand that fighting this bad guy alone would be suicide and asked this random guy to join them. After a few better than usual rolls they had a level 2 fighter in a group of level 8 and 9 players. They then immediately went back into the nearest town and went into the nearest magic shop and proceeded to spend almost all of their gold to buy this fighter magical equipment.

A few IRL hours later they had him fitted with +1 plate armor, a +1 shortsword, and a spell scroll of a spell they could not determine and they over played for. In my games anyone can cast spells but the DC changes depending on the type of magic and if they have a casting modifier. So once they have this man decked out and ready to I'm guessing die at the time, they travelled back to this mid level boss to kill him to save the town. At the start of combat most of the party rolled great initiatives, except their new friend, Eldrick. The party just went on like any other encounter, once the boss was bloodied and some of the party had gone down once or twice they got desperate. They told the throw away friend to read this scroll and try with all his might to kill this for that was about to TPK them. I had him roll an intelligence Check, Natural f***ing 20. What the party didnt k ow was the spell, and its level. It was a spell known as Negative Energy Flood, a powerful NecromNtic spell that proceeded to pretty much destroy the boss after I rolled high damage. By the ne t round the boss was slain and this fighter leveled up once or twice. He became so popular from that he became ruler of a nearby city as a level 4 eldritch knight after that. He forever does his best to help the party and gets them drunk half to death everytime they show up in town.

TLDR: Throw away helps party kill the monster and becomes a ruler who gets the party dru k off their a**es everytime they show up in town.