ToME: the Tales of Maj'Eyal

So, I used the Wyrm Bile on a Necromancer and I understand that the stats increase and in this instance, it increased my magic by 5 but then... my spellpower went up on the character sheet by 9!!

That's an error or is that an effect of the Bile?

It is most likely the effect of something else that you didn't notice.
Wyrm Bile should have no effect on Spellpower outside of the minor stat changes. (Unless you spend the category point on something that then effects Spellpower.)

Powers get more and more "expensive" to raise (capped at 5:1 iirc) . Hence the differentiation between Spellpower and effective Spell power.