But what happened to tathamet's "soul"? Where Anu's soul transcending reality, I dont believe it stated what happened to the Dragon's essence. Flash forward to present day. In Diablo's latest incarnation, it has been speculated that he (or she as it might be) has actually been fused with his brothers, Mephisto and Baal, as his latest form seems to incorporate aspects from all 3 prime evils
I have a feeling that the Great Evils, Diablo, Mephisto, Baal, Duriel, Azmodan, Belial, and Andariel are actually all being directed, at a subconscience level, by the "spirit" of Tathamet, and all their actions, the very reason they are at war with the Heavens, is for some way, artifact, or place that will allow the great evils to once more fuse together to form Tathamet.