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.