Being sealed close to the surface, time wore away at his body, leaving his skin so rotted that he covered all of himself in a shroud, his face a grinning mask. Further, he was no longer able to speak.
In "The Wheel of Time Companion" his strength level is described as ++3, one of the highest levels known, but still below the Dragon and some other male Forsaken.
Eval Ramman had a bad temper. That, combined with his great strength in the One Power, almost earned him a sentence of being bound with the Power to keep him from violence. He was also fond of women and gambling, cheating in the latter to impress the former. Lured by the promise of endless life, the historian was easily swayed to join The Shadow during the middle years of the Collapse.
During his time as one of the Chosen, he ran an extensive intelligence network, bested only by that of Moghedien, and committed several great atrocities - for example, he raised and organized breeding camps for humans to be fed to the Trolloc armies.
When Lews Therin Telamon and the Hundred Companions struck at Shayol Ghul to seal the Bore, he was there along with twelve other of the most powerful Chosen, and imprisoned for three thousand years before his release in 998 NE.
Soon after his release, he and Aginor confronted the Dragon Reborn, Rand al'Thor, at the Eye of the World. After manhandling Nynaeve al'Meara, Balthamel was confronted by Someshta who would not allow them to cause destruction in his home. Balthamel burned the Green Man badly, striking a mortal blow to the all but immortal being, but he, himself started to rot as organic weeds, fungi and such grew all over him from the touch of the Green Man, subsequently killing him.
Later the Dark One reincarnated Balthamel in a new female body, calling him Aran'gar.