So how would you balance a new curse in terms of power? Just because
its a minor inconvenience to most doesn't mean its not devastating to
Take your cue from the examples. They are limited in scope so try to impose similar limits on new curses. Just as it says in the spell, "You may also invent your own curse, but it should be no more powerful than those described above."
I would suggest the following guidelines:
- No single penalty is greater than -6.
- No across-the-board mechanic penalty is more than -4.
- Total failure is not an absolute, at best that only happens 50% of the time.
So there is a set of limits you can point to, taken from the examples. Using those, your could have a single mechanic roll, say all save rolls, have a -6. Or say 50% of all NPC reaction rolls fail.
For instance, you make someone slur. For most its nothing, but for a
spell caster with a verbal component, terrible.
That's a good one. Keying off the examples, I would implement this a great deal like the third example. So, on any spell casting with a verbal component, 50% of the time it fails due to the slur.
Would ill fortune (on d20 rolls, roll twice and take the worse result)
be a fair curse?
On all d20 rolls? No. Again, limit it like the examples are. Say two or three types of roll, skills, or saves, or random encounter rolls, or reaction rolls, or treasure rolls, you get the idea. Target the "ill fortune" a little more than all d20 rolls.
Just to throw it out there, one of my favorite curses of all time, note this was e2 AD&D, was any critical hit does minimum damage. When it hurt, it hurt, but it was very focused in effect so it wasn't over powered.