ToME: the Tales of Maj'Eyal

I tend to play characters worshipping Melkor, with long term plans to destroy the Ring and switch to Eru..

Main reason for worshipping Melkor is access to Udun spells, specifically Genocide and Drain..
And Curse, ofcourse..

But, since an alchemist can make his own scrolls of (mass) genocide, and potions of restore mana..
And I find that essences of magic are relatively rare, and needed to make scrolls of *identify*..

Would starting out with Eru be a better choice, for the access to Mana school and the Music spells?

Once you get 50 attacks with a +50, +50 vampiric blade of chaos of westernesse, have 7000 hit points from your +800 % to life quad-immunity armor, and are moving at +60 speed, the god you are worshiping is irrelevant.

Before you get to that stage it could matter. You should pick a god that suits your game play and makes it more likely that you won't die prematurely. Depending on your approach, any one of them could be useful. I have even played an antimagic alchemist.