These kinds of fan music videos are hit and miss for me. Most of the ones I come across aren't that good, but Game of Thrones - Ultimate Birthday Rap Battle is actually done pretty well. It's set up at a 10-year-old's birthday party, which doesn't make much sense, but that's the vibe they were going for. Here's a note from the team that made it:
Straight out of Westeros comes the "Game of Thrones" Ultimate Birthday Rap Battle! It's Max's 10th birthday party--there's cake, candy, balloons, a bouncy castle, and the characters from "Game of Thrones" with their medieval swagger on full display. Epic rap breakdowns from all of your favorite characters! Including Robert Baratheon, Jon Snow, Robb Stark, Joffrey, Khaleesi (played by Taryn Southern!), and more! Watch how a birthday party is supposed to be—Seven Kingdoms-style—in the most ultimate rap battle ever!"