Original download link:http://www.moddb.com/games/spore/downloads/spore-creature-stage-level-editor
NOTE: This mod is extremely buggy, and will probably crash a lot.
Also, on the website I got this mod from, there’s a comment saying that this gave them a virus. It didn’t give me a virus, though.
I played around with it a bit, and it works. To enter the level editor, enter creature stage and swim out to sea until you get to the point where the sea monster would normally kill you. Instead of the sea monster, though, you will be greeted with the level editor. I haven't tested it much so far, but from what I've tested you can add creature nests (you can change the size of the creatures to make them half the size of a planet or the size of a stick, you can make nests of your own creations, you can make creatures that have no nest, and you can make different "types" of creatures (like rogues and epics) and rocks. The editor has a bunch of other options like SAVING AND LOADING PLANETS(The load feature crashes the game for me, and I haven't tested the Save feature).
THE LOAD FEATURE ACTUALLY WORKS (For most planets, when you attempt to load some it crashes your game. It seems like only T0 planets can load properly). It seems like the Save feature does nothing. You can't terraform planets using this editor, but there might be a way to terraform planets in space stage and import them into the editor? I have no idea if you can play the creature stage on planets changed in this editor.
If a bunch of people already know about this, let me know and I'll delete this post. I didn't see any posts like this, so I posted it just in case.