[RPG] Should this homebrew “Artillery Spell” advanced metamagic option be reworded or clarified

How many Sorcery Points should this cost? Lots. At least 5, since this metamagic is better than Careful Spell, Empowered Spell and Heightened Spell combined. I'm sure you can see where I'm going with this - it's too powerful to put a realistic cost on it.

Your new metamagic option does 3 things. The first is pretty simple - it multiples damage by 1.25, or 1.5 if it's an AoE spell. I'm going to look at Fireball (a pretty classic damage spell) as a standard example.

A Fireball does 8d6 damage, averaging 28 damage. An Empowered Fireball, assuming a Charisma of 20, does 8d6 with the option to reroll up to 5 dice, for an average of 31.75 damage. A Sharpened Fireball does 8d6 * 1.5 damage, for an average of 42 damage.

At this point, you might be thinking "But Fireball does 8d6 damage, where the entry in the Spell Damage table for a 3rd-level spell is only 6d6!" Unfortunately, the advice in the Spell Damage table bears little or no resemblance to the spells that were actually published.

Note that this is also the case for single-target spells. Disintegrate (which uses a saving throw, not an attack roll) deals 10d6 + 40 (average 75), Empowered Disintegrate deals 10d6 with up to 5 rerolls + 40 (average 78.75), and Sharpened Disintegrate deals (10d6 + 40) * 1.25 (average 93.75).

At any rate, hopefully you can see how Sharpened Spell is considerably more powerful than Empowered Spell.

Moving on, the second thing Sharpened Spell does is change the target making a saving throw into the caster making an attack roll. Ostensibly, this is a loss in damage, since most (though not all) spells that use saving throws deal half damage on a successful save. However, it is far easier to make an attack roll hit than it is to make an enemy fail their saving throw.

Firstly, it's a trivial matter to give yourself advantage on an attack roll; it's an extremely non-trivial matter to give an enemy disadvantage on a saving throw. For example, you have inspiration, True Strike, the Help action, the Hide action, The Wild Magic Sorcerer's Tides of Chaos, and heaps of other things to give yourself advantage on a roll. To give disadvantage, you have Heightened Spell, or Contagion. There's not many, and they require significantly more expensive resources than the advantage options.

Secondly, most powerful monsters (the kind that would be worth sharpening a spell for) have Legendary Resistance, which makes it impossible to make them fail their saving throws, since they can just say they didn't. You can make them burn through their uses of this feature, but if they're smart they'll only use it on your best spells.

Finally, attacks are (by design) easier to hit. Everyone uses attacks, and some characters use them exclusively, so monsters can't be impossible to hit without making some players feel useless. On the other hand, not everyone uses saving throws, and everyone can still function without them. It's difficult to prove this mathematically, but anyone can quickly observe that there are extremely few monsters with AC above 20.

A mid-level sorcerer (let's say level 8) with maxed Charisma only needs to roll 12 or higher to hit an AC of 20. With advantage, which you'd better believe they'll find a way to get on their Sharpened Fireball, that's a 69.75% chance to hit.

The same sorcerer casting a regular Fireball has a save DC of 16. Let's assume the monster isn't great at Dexterity saving throws (but not too bad at them, either) and give it a +3. It has a 60% chance of failing that saving throw, which is pretty good odds for the sorcerer, but not as good as the attack roll. On the other hand, if the monster gets advantage on the saving throw (which there are a lot of ways for it to do), it has a 36% chance of failing. Not so good. And, of course, this is assuming a monster with really good AC, which is more of a worst-case scenario than a usual one.

So that's why Sharpen Spell is also better than Heighten Spell. The third thing Sharpen does is change the spell from an AoE to a single-target. This looks like a drawback, and often will be, but in those situations the sorcerer just won't do it. For the sorcerer to be thinking about Sharpening a spell, they'll have a priority target in mind. This lets them use Fireball on the Big Bad without having to worry about hurting their allies. "But they could just use a single-target spell!" Well, they could. But they have a limited number of spells known, and there are in fact no 3rd-level single-target spells. Hence, this is also better (or at least as good as) Careful Spell, albeit only when you have a priority target.

With all that out of the way, this is a really cool idea. It gives us back the Spellwarp Sniper from 3.5, which was great. Focusing Fireballs into concentrated beams of destruction? Awesome.

I strongly recommend going ahead with this, but removing the damage bonuses. If your players choose to use this, they know they're turning an AoE spell into a single-target spell, and they know they're adding a chance of total failure. They'll use it when those aren't problems, so you don't need to boost it because of those drawbacks.

With that change in place, I'd recommend this metamagic option costing 3 sorcery points, the same as Heightened Spell. Wait, what? I said earlier that it was better than Heightened Spell! Well, it is, but it requires the player spending additional resources to make it that way. It's trivial to give yourself advantage on an attack roll, but it's not free. You have to spend an inspiration point, or use your previous turn to cast True Strike or to Hide, etc.

Formatting

I think below may be a better way to phrase things to sync with 5e's standards:

Thoughtseize

Casting Time: 1 action [I assume]

Range: 30 feet

Components: V, S, M (a needle or pin)

Duration: Until the end of the target's next long rest¹

You probe the mind of your target, and can sever a thread of their memory for a time.

Choose a creature within range. The creature must succeed on a Wisdom saving throw.² On a failed save, the affected creature forgets one spell from their spell list of a level lower than the spell slot used to cast this spell (cantrips count as level 0). You may choose which spell is forgotten.³

When you cast this spell using a spell slot of 3rd or higher, you cause the target to forget another spell for every two slot levels above 1st.⁴

If the targeted creature is currently under the effect of this spell, its previously-forgotten spells are restored to its memory (unless chosen again).

This effect lasts until until the affected creature completes a long rest.⁵

Specific Comments

I've provided footnotes at each point wherein I have comments on the spell:

1. The spell's duration is a bit unusual, but in general, as written, lasts until the end of the creature's next long rest. I would note, this is a very long time, which is unusual for a 1st level spell and is potentially an unbalancing element for it. The Warlock's Hex will only last a full 24 hours with a 5th level spell slot. I would recommend consider tying the duration to the spell slot and probably start with something like 1 hour.

   I noted that this spell does not require Concentration to maintain the effect. This also gives it a lot more power than perhaps initially intended. As the presumed purpose of this spell is to shutdown an enemy caster, I think it needs to be balanced by including the Concentration mechanic as this is typically the balancing mechanic for spells that shutdown an opponent. The spell has an inherent ability to shutdown an enemy caster by taking away their attack cantrip, which for low level creatures can force them to using daggers or unarmed strikes or can have a serious impact on certain legendary creatures whose legendary actions are tied to their cantrips.

2. Your original description indicated that the Wisdom saving throw was equal to the caster's spell save DC. This is unnecessary language; all spells use the caster's save DC unless they specifically say they don't.

3. This seems very powerful. As written, it can be inferred that the caster gets to view the full spell list of the target. You may wish to use language that constrains the information provided to the caster to only what they can cause to be forgotten.

4. Your original description stated 'each second level after 1st'. I was unclear if the intent was an additional spell for each spell slot above 1st or for every 2 spell slots above first. Regardless, for balance reasons I would recommend using the latter otherwise the caster of this spell is likely to completely shut down an enemy caster with a single upcasting of this spell that takes away nearly half of the enemy's spell list.

5. Your original language indicated that this effect lasts until dispelled or the creature completes a long rest. I don't think the language regarding being dispelled needs to be included as that is true of any long-term magical effect.