We demonstrate that the intricate energy spectrum of neutron-rich helium isotopes can be straightforwardly described by taking advantage of the low-energy properties of neutron-neutron interaction and the scale separation that is present in diluted dripline systems. By using arguments based on the halo effective field theory, we carry out a parameter reduction of the complex-energy configuration interaction framework in the spd space, including resonant and scattering states. By adjusting only one parameter, the strength of the spin-singlet central neutron-neutron interaction, we reproduce experimental energies and widths of 5−8 He within tens of keV precision. We predict a parity inversion of narrow resonances in 9 He and show that the ground state of 10 He is an s-wave-dominated threshold configuration that could decay through two-neutron emission.Introduction.-The neutron-rich helium isotopes 5−10 He epitomize novel aspects of nuclear structure at and beyond the limit of nuclear binding. Experimentally, the even-even isotopes 6 He [1] and 8 He [2, 3] are Borromean halos, they have no bound excited states, and they exhibit an abnormal pattern of the one-and two-neutron emission thresholds. The odd-N isotopes 5 He [1,4,5] and 7 He [6,7] are neutron-unbound. Presently, too little is known about the elusive 9 He [8,9] and 10 He [3,4,[10][11][12][13][14][15][16][17][18][19] isotopes to firmly conclude whether they represent genuine nuclear systems or not. The current experimental information on the energy spectrum of 5−10 He is displayed in Fig. 1.