One of the current theories is that Tarinax is Tarinax.
Nothing proven or elaborated upon, but much like how Siyamak and Barakah can hide themselves under the disguise of an illusion, a few of us believe that Tarinax spent a lot of time disguised as an orc. His soul was taken by the Shadow Men at some point. The Orcs never knew he was a dragon. They just think they're getting the soul back of some great orc from the past. Swearing loyalty to Kerafyrm, even without a soul, he is quite contempt with guarding Deathtoll for the rest of eternity as the undead.
To the contrary, however, we know that the shadowmen have been trying to obtain Wuoshi's soul so they can inhabit his body and better observe the world in thier ongoing plans to wipe Norrath off the face of the uni-dimensional map. There is no mention of any previous attempts of taking over the soul of a dragon. The possibility exists that not even the shadowmen knew that Tarinax was indeed a dragon.