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
one.
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.
Concentration is for the duration of the spell
The 1 minute duration of the spell is for both holding the spell until you deliver a hit as well as for maintaining the Condition effect of the smite (in this case, Frightened condition.)
This is supported by Jeremy Crawford via Sage Advice
Some smite spells, such as wrathful smite, do have an ongoing effect that requires concentration
Best Answer
No, the effects don't stack over the same duration
According to the rule on combining magical effects (PHB, page 205), the effects of different spells add together but the effects of multiple instances of the same spell do not.
Instead the most powerful version of the spell would from the casting. If they were the same spell slot level used, then it would be up to the DM in this case to determine which one is active on a creature.
This assumes the creature fails both saves and what you are going for is for the creature to make disadvantage wisdom saves to be able to take an action.