Well we're not in canon, there wasn't any death trooper that believed in The Emperor like a maniac, our characters didn't exist. There was no Heavy Trooper Named Di that killed Jedi. It doesn't have to exist in the Star Wars universe for us to use it. The whole planet will be made up so why not add this artifact? The rebels could have secretly build a base on this planet, hidden for many years, just to study the structure. And as we pass near the planet they want to try out it's power for the first time. We're the practise target. In the end its about having a fun and unpredictable adventure.

Also, they built a Sun Crusher. What's the problem with a pyramid shaped structure destroying one Imperial ship that happens to pass near the planet?