Created by Steve Duda (the mind behind Serum), Cthulhu is actually : Reddit·r/TechnoProduction
: Widely considered the best free alternative. It is a MIDI plugin that lets you map complex chords to single keys, much like Cthulhu's chord module.
: A great lightweight VST (Windows only) that triggers full chords from single notes. It includes many built-in scale templates.
: Xfer Records offers a trial version for both Windows and macOS. It is fully functional but times out after 15 minutes of use, though you can simply reload it to continue experimenting.
: If you already have the demo or the full version, many sites offer free chord packs to expand your library. AngelicVibes and Red Sounds frequently release free "Bach Chorales" and modern Hip-Hop/EDM chord progressions. Top 3 Free Alternatives to Cthulhu
If you need a permanent free solution for generating chords and arpeggios, these community favorites offer similar functionality: