Master of Magic/Caster of Magic
I want to know how SP skill production works, because when I cast Drain Power the casting skill only goes up by 1 point. MOM/COM is a great game - it's a shame that monsters can't get experience points too and toughen up or evolve into tougher creatures (for e.g. like a highly experienced demon lord)

https://masterofmagic.fandom.com/wiki/Spell_Casting_Skill#Power_and_Skill_Point_Summary

Life Drain (not Drain Power, that's something else, at least in MoM) gives the equivalent of 3 points of power spent on skill, per point of damage dealt (i.e. per point of resistance the target failed its roll by). It is unlikely that a single cast ever yields more than 1 skill increase, if that, considering that to improve skill by one point, twice the current value in power needs to be invested.

I was talking about Caster of Magic more, but is it true that you have to invest double the current skill points to increase the casting skill? On that page it had an example of it.

Spell skill is handled a little differently under the hood, but that formula is more than close enough that you don't have to care about the actual implementation if you don't want to.

The higher your spell skill, the more you have to invest to increase it further. Life Drain can help increase your spell skill, but that is a very minor benefit. Life Drain's primary purpose is as a damage spell with a chance at getting an undead unit afterward (if drained life is the primary way the victim dies).

I haven't checked the DOS version of CoM, but in the windows version, skill = sqrt(SP), which yields an increment that is 1 point more than double the current value. That's close enough. At the same time, in both versions of CoM, Drain Power does indeed grant SP for each point of mana drained, which is 180 + 3% of the target's reserves. That seems like a lot initially, but if you have a high skill already, it probably isn't.