Board Thread:Warhammer 40k General Discussion/@comment-24332685-20140111015135/@comment-24944355-20140716112447

None of the surviving Primarchs could take their Demon Prince brothers without help, Guilliman proved that when he was overwhelmed by Fulgrim. That theory was also likely proven, if as I believe, by Mortarion killed the Khan & Perturabo killed Dorn.

After all it took over 100 Grey Knights to defeat Angron & most of them died in the process, it would take far fewer Grey Knights to defeat any of the undemonised primarchs.