I tweak the file of my sovereign to add the life spellbook. It now appears in the least of sovereigns, but it gives no new spells, at least at level 1... Searching for the code corresponding to this spellbook, I found that the spells are there. It makes it a very minor graphical bug... Supposing the spells you get depend on the sovereign's race or faction you choose. What if I wanted the death spellbook? Well, I don't, but...

Which is lame.

There's no reason custom sovereigns couldn't be the same as premade sovereigns. Already Kul-an-something is worth way too much in comparison to what can be built, which is annoying( play MP and pick him while someone else creates his own sovereign...).