Disclosure: Key provided by GOG. GOG links are affiliate links.
Here's what it adds:
- Hunt and eradicate 5 different species of Celestial Behemoths to obtain Precursor artifacts
- Upgrade your Precursor ship using the collected artifacts
- Hunt and eradicate 5 ancient Behemoth Elders for powerful trophies and lore
- Boost the power of your fleet by equipping Elder trophies onto your ships
- Start your battle against the Chruul on the right foot by adding the Precursor ship to your fleet
- Upgrade your Precursor ship all the way to TIER 6!
- Unique New Precursor powers can reduce enemy resistances to make combos easier
It's been quite some time since I last played it, but with the Lightspeed Edition and the DLC together it's got a lot to offer. Some interesting gameplay that mixes base-building like seen in the newer XCOM games, turn-based combat both between space ship and people on the ground, exploration and plenty more. It also has some rather eerie music to go along with it.
The base game is on sale on GOG at the moment, with ~9 hours left on the sale you can grab Halcyon 6: Lightspeed Edition with 49% off, the DLC is not on sale but it's already damn cheap.