ToME: the Tales of Maj'Eyal

I just found it on the bottom level of the Old Forest.
It's description says: 'This potion fundamentally twists your physical features, as if you were another mother's child. Your physique will be shaped anew'

What does it affect? Stats? Height / weight? Race?

You have an hp 'score', expressed as */100 and the potion rerolls this value. I believe it only affects your hp before constitution modifiers, life multipliers, etc, so the potential benefit or loss to your health is fairly minor. You can (again, IIRC) see your current */100 by drinking a potion of *Self Knowledge*.

Sorry for the vagueness of the answer, but it's ages since I've played this game now and I can't recall all the technical details

Not too vague at all, you make sense.
Now to find a potion of *self knowledge* to add to my recipes.
Phew! 16 essences of Extra Life for a potion of new life. Expensive.

These are useful if you find them in a fountain, otherwise I don't bother to drink them. Seems to be about a 50% chance of decreasing your hp. I've used them in Erebor to gain ~150 HP. It is minor, but then I head on over to Melkor's altar......

Hmm.. Yes... I am at level 29 now, with maxed out stats, CON at 40.. I have 630 hitpoints.

When sacrificing life to Melkor, is the bonus you gain permanent?

I know they're permanent at least as long as you're worshiping Melkor. I don't remember what happens if you stop worshiping or destroy the ring.

If you are playing an alchemist, you want to extract it for the essences, if I remember correctly.

Otherwise, sell it. It is worth lots of money.

Every character has a "life rating" that ranges from 80/100 - 120/100. If you drink the Potion of Self Knowledge, you can find out what your life rating is. If it's low, then you can risk drinking the potion to see if you can get some more health out of the deal. Depending on where you are in the game and what your life rating is, it can have pretty big effects on you.

Hehe.. Yeah, I am playing an alchemist. The potion is already reduced to it's components. I'll see if I need to make some anew once I find (the recipe for) a potion of *self knowledge*