Here is the full list of all 44 Stellaris Nexus achievements.
Win a Succession match.
Finish a multi-player Succession match.
Complete the Story Mode.
Win a Succession match as the U.N.E. Patriot.
Win a Succession match as the U.N.E. Diplomat.
Win a Succession match as the U.N.E. Admiral.
Win a Succession match as the Voor Technocrat.
Win a Succession match as the Voor Teacher.
Win a Succession match as the Voor Explorer.
Win a Succession match as the Kel-Azaan Aggressor.
Win a Succession match as the Kel-Azaan Hunter.
Win a Succession match as the Kel-Azaan Conqueror.
Win a Succession match as the Turtuon Statesman.
Win a Succession match as the Turtuon Monumentalist.
Win a Succession match as the Turtuon Nationalist.
Win a Succession match as the Chinorr Profiteer.
Win a Succession match as the Chinorr Exploiter.
Win a Succession match as the Chinorr Mercenary.
Win a Succession match as the Ix'Idar Collective.
Win a Succession match as the Ix'Idar Proliferator.
Win a Succession match as the Ix'Idar Infiltrator.
Win a Succession match as the Yondar Crusader.
Win a Succession match as the Yondar Inquisitor.
Win a Succession match as the Yondar God-Queen.
Win a Succession match as the Roderian Overseer.
Win a Succession match as the Roderian Mechanic.
Win a Succession match as the Roderian Instigator.
Win a Succession match as the Glebsig Visionary.
Win a Succession match as the Glebsig Socialite.
Win a Succession match as the Glebsig_Protector.
Win a Succession match as the Kilik Progressive.
Win a Succession match as the Kilik Marshall.
Win a Succession match as the Kilik Bureaucrat.
Complete story Mission 1.
Complete story Mission 2.
Complete story Mission 3.
Complete story Mission 4.
Complete story Mission 5.
Complete story Mission 6.
Complete story Mission 7.
Complete story Mission 8.
Win a Succession match as the Ave'brenn Exalted.
Win a Succession match as the Ave'brenn Fanatic.
Win a Succession match as the Ave'brenn Seeker.