Rand starts gathering men who can channel (Asha’man), and builds the black tower. The first man he brings to his cause is Mazrim Taim. Taim is a very strong channeler, and close rival to Rand (much like Demandred was).

There is a lot of friction between Rand and Taim as Rand gives him license to train men, and essentially anoints him to the male equivalent of the Amyrlin.

Friction continues, Taim starts training his hand picked guys who are loyal to him showing them stuff he doesn’t show others.

Eventually Taim is shown as a dark friend.

The whole way RJ is hinting that Taim is Demandred but he changed course