He was a stout man with thin gray hair.
At the end of 999 NE Rand asked Fel first about the Warder Bond and if it can be broken and then about the seal on the Dark One's prison, most notably how it would be possible for him to repair it during the Last Battle. Fel pointed out an inherent contradiction, namely that the prison needs to be completely intact when the Wheel of Time turns and the Age of Legends comes again, so the Aes Sedai of that time can create the Bore once again. He also mentions that the Dark One needs to have been forgotten by this time as well.
Fel became intrigued by this contradiction and began thinking on how it might be resolved. He reached a possible conclusion and wrote down a cryptic phrase "clearing away the rubble". At this point, he was brutally killed by the gholam in his study, ripped apart limb from limb, suggesting that the Shadow was aware of his line of research and considered it extremely dangerous. Recent evidence as unearthed by Min Farshaw has suggested that "clearing away the rubble" means destroying the seals so that they can be rebuilt.