Straight from the official release thread on Hard Light Productions:
Inferno: Nostos Act 1 released
The Inferno Team is pleased to announce that, after fifteen long years, Act 1 of Inferno: Nostos is now available on Knossos.
A recent nightly build (20180506 or newer) and the 2014 MediaVps
NOTE: Knossos can be configured to download recent nightlies by setting "preferred engine stability" in the Knossos tab of the settings menu to "nightlies." Update your SCP "mod" in Knossos to download the latest nightly build, then select it as you would any other build.
Available now on Knossos, or via a direct download:
Manual install links are now available from FreespaceFiles and Nebula:
INF_Core: Nebula (no mod.ini) FSFiles
INF_Maps1: Nebula FSFiles
INF_Maps2: Nebula FSFiles
INF_Maps3: Nebula FSFiles
INF_Maps4: Nebula FSFiles
INF_Models: Nebula FSFiles
INF_Effects: Nebula FSFiles
INF_Audio: Nebula FSFiles
INF_Radaricons (optional): Nebula FSFiles
INF_CBAnims (optional): Nebula FSFiles
The first 15 missions covering the first half of the EA war to the arrival of the Shivans in Ross 128! Experience the Earth Alliance War like you've never seen it before!
Countless new ship types, and many R1 classics are back and better than ever!
New HUD interfaces, effects, sound, and music!
And of course, big, epic, badass space battles.
Does it require the old Inferno R1 release?
No. This replaces that release.
What happened to X model?
Some assets are being held back for story reasons with more models being released with Act 2. Act 1 includes around 75% of the final modpack for the entire campaign.
How many acts will there be?
Either 3 or 4 depending on how development of the final major models goes. The next release will go up to the end of the EA war.