Adds a new planet, tenebris, a dark planet enshrouded in a thick acidic atmosphere.
Mods introducing new content into the game.
Changes to enemies or entirely new enemies to deal with.
Map generation and terrain modification.
New Ores and resources as well as machines.
Things related to oil and other fluids.
Furnaces, assembling machines, production chains.
Changes to power production and distribution.
Can this be added to an existing save game? (currently playing SA and I'm still on Nauvis but close to reaching Fulgora).
Yes, should be fine.
You don't get access to this planet until after Aquilo anyway.