To get Polyatomic Ion Particles in The First Descendant, you can kill any mission monster on Fortress. Mission monsters are the powerful yellow bar enemies that can spawn in during missions.

A polyatomic ion that was thus far inaccessible with conventional mass spectroscopy has now been uncovered using extremely short laser pulses. Catching a glimpse of this ion would open up new ...

Polyatomic Ion Particles can only be found in the Fortress Region. To get them, you need to participate in missions and kill enemies with yellow health bars (aka boss monsters).