Hi everyone, that's my first post here, btw.
Alexander is just an ordinary human being, with an extraordinary esoteric knowledge. He might know some kind of tonic/potion/alchemist stuff/whatever... to keep him alive during many centuries, but he longs for something more: going back home.
But this home is not some special place where he, and only he comes from. It his (in his belief) the place where all human souls come from, before falling in the world of generation.
The key to that is given by Agrippa speech, which refers to mitraism. The eschatology of this ancient religion consisted precisely in escaping this lower world by climbing a cosmic ladder across the seven planetary spheres, in order to reach the eighth gate of the ultimate sphere.
And this sphere is the one of the fixed stars, the otherworld laying beyond the last gate is "beyond the stars" like the otherworld where Alexander is longing to go.
Look in the so-called "secret room" in the chancel and you'll find a statue of Mitrha killing the bull. Mithraism explains the very logo of Amnesia. Mithra was conceived as and Unconquered Sun, and Other Sun, an Inverted Sun, a Black Sun, a True Sun... Another Sun ruling the mundane Sun, and the whole cosmos, from the shadows
outside the cosmos.
So in the A TDD logo we have an inverted sun, and three planetary symbols (Jupiter, Saturnus, Neptunus) referring to the cosmology of the planetary spheres.
"The alchemical process is the Mithraic Mystery, the incommunicable Mystery."
- Zosimus of Pannopolis
Mithra holding an orb/globe, the sphere of the cosmos.