The "task defeated" sfx actually made me jump haha. I really like the concept for this idea! I've tried a few game-ified task management apps (I even use one regularly) but this one really takes that to a new level.
One thing that I think could work really nicely is time stamps. For tasks, that could be the time / date that it was added, so older tasks aren't left behind. The defeated list could also show the time / date that each task was completed. The ability to sort the task list by time could also help keep the list organized in different ways (as well as inverse-sorting, where higher-priority tasks are placed on top).
Overall, this was a very novel concept. The UI was very intuitive, and I don't feel like any necessary functionality was missing for this to be its own fully-functional task management app. Great work!