This is about more than just lore, lore is involved.
Wave 1:
Achievements
there are many ways to add achievements to the game
however, here's how i see them working
A small menu for achievement would be shown, where you can see things you've done, at the top, there would be basic stats (money, leaderboard spot, etc, etc), then, there would be the other achievements or special things.
Examples:
- Welcome! (play Swordbattle for the first time)
- Just getting started (Get your first kill)
- One of many (Die for the first time)
Obviously, that wouldn't be it, that's just some examples. But the fun part is where the lore could come in
The achievements could have (enlargeable) images that could show simple things, like a skull for the One of Many achievement. however, some might show bits and pieces of lore. These lore drops would most likely come from moderately difficult or secret achievements. also, as an extra thought, some of the insanely hard or at least hard achievements might contain rewards like skins.