All that is known of Culan Cuhan is referenced in Rand's memories in the fifth book when he speaks of Sammael turning to the Shadow and becoming known as the Destroyer of Hope.

Sammael razed the Rorn M'doi and betrayed the Gates of Heaven. These actions Rand remembers made Culan Cuhan weep. It is suspected that Culan was a strong and prominent figure or leader or some capacity, perhaps a General or a male Aes Sedai whose tears were shocking enough and unexpected when he shed them to be remembered.

There is also the equal but less likely possibility that Culan was a powerful member of the Shadow, whose cruelty was legendary, to emphasize the magnitude of the great destruction and betrayal when Sammael turned.

His name might potentially be a shout-out/throw away parallel of Cú Chulainn, Irish mythological hero.