A human cook on a space mining ship finds an immortal alien when exploring a cave on an asteroid. The alien is the last of its kind. I think the others of its species may have been accidentally killed in the mining process. The alien becomes the man’s companion. It can change form, and heal itself and others it touches. In addition to changing its own shape, the alien can also envelope the man to change his appearance too… and it extends its healing power to effectively make the man immortal.
I think thousands of years pass before the protagonist of the novel is introduced. He teams up with the (now immortal) character. They have a mission of some sort but I don’t recall what it was. As the main plot unfolds, it is eventually revealed that the immortal man (using the powers of his alien companion) has been shaping human societies development across multiple solar systems for his own reasons. Society is decadent and corrupt and it turns out the immortal baddy is both the ruler AND the mysterious leader of an underground revolutionary group. I think the whole thing was for his entertainment rather than any justifiable reason. In the end, I think he is killed and the alien/symbiote teams up with the protagonist.
I don’t recall the plot of the main story but the villain pretends to be helping protagonist, until the big reveal.
I recall that the society (which has been shaped over millennia by the antagonist) is decadent and corrupt. The humans are fat, wear makeup, and enjoy watching blood sports. There is a scene where giant dogs are ridden on a hunt of some sort. The central goal of the protagonist may have been to destroy a tower that was somehow essential to interstellar travel.