So to the Hazbin Hotel fandom… I know it’s not going to be what fandom wiki says for the show, but there is something on there about who/what has Alastor’s soul. And with his magic being toxic green, I’m a little more prone to think it’s true based on other media.
Alastor sold his soul in hell, not before, in this belief, so please bear with my hypotheticals.
Alastor made it to hell and they say he just appeared, but I believe he was there just in weaker form at first. That is what his deal is about at first. So he signed up for Voodoo, now bear with me; I know in his living lifetime he wouldn’t have signed off on that for power (having been told by his Mother to be careful with it), but dead Alastor would after becoming semi-powerless (he was a powerful serial killer). Oh he absolutely would.
This plays into his seven year disappearance too. He had to pay his debts. Especially for the amount of energy he was using. So to come back, he made another deal; get Voodoo in with the Morningstar family/get Charlie’s soul for Voodoo purposes.
As for why I believe this; the finale song- we see the bonds of the deal on him. Toxic green stitching- stitching that is reminiscent of a Voodoo doll and it’s not just on his face- it’s on his sides too.
Sadly I cannot find the image I want when I look for stuff about Alastor in the finale song, but we get a look at the stitches around his arm in his deal with Charlie, so that will have to do. To be fair I also didn’t notice all the symbols around the building when she made that deal, but I don’t know enough about them/don’t know if they were just add in for a bit of design.