Warlocks have spell slots that scale with their level. Usually most spells that have an upcast feature either deal extra damage (they seem to follow the DMG guidelines for damage/level) or target extra creatures. Conjuration spells may summon extra creatures and healing spells heal more but that also relates to damage.
Are there any spells with more unique or powerful effects (even if just dealing more damage than expected when scaled)?
Not necessarily from the warlock spell list.
This spell has a highly unusual upcast description:
At Higher Levels. If you cast this spell using a spell slot of 4th level or higher, the duration is concentration, up to 10 minutes. If you use a spell slot of 5th level or higher, the duration is 8 hours. If you use a spell slot of 7th level or higher, the duration is 24 hours. If you use a 9th level spell slot, the spell lasts until it is dispelled. Using a spell slot of 5th level or higher grants a duration that doesn't require concentration.
Not only does the duration grow pretty rapidly, it eventually reaches "until dispelled", and at 5th level it turns the spell from one that needs concentration into one that does not.
We discovered, during play, as my bard used this spell to debuff various enemies, that blinding a lot of enemies by upcasting blindness on them (with a high enough spell save DC, it was striking how often the save was missed) really made the battlefield change. I upcast to levels 4, 5 and 6 on a number of occasions in Tier 3 play. A couple of great aspects to the spell are that: (1) it does not require concentration and (2) if the enemy can't see the party, there are a whole host of spells and abilities that cannot be used until the save is finally made.
That it gives allies advantage when attacking the blinded opponent was a key reason I'd use it, since I was playing the bard as a support caster.
I saw similar results with blindness as a DM when the cleric player used it as a debuff.
While it was occasionally useful at lower levels, blindness became a lot more useful as my bard's Spell Save DC went up and she could impact more enemies. Unlike hypnotic pattern, it didn't have the "charmed condition" rider that makes that nice debuff not useful versus creatures who are immune to the charmed condition. (As CR goes up that number seems to increase).
Two other spells that I see upcast with some frequency are fly and invisibility. Upcasting provides the benefit to more creatures, which can tactically have a substantial impact, and both of them can also have an impact outside of combat. (10 minutes of flying gets you over a non-trivial number of obstacles, the whole party being invisible for an hour allows for quite a bit of movement, infiltration, evasion, etc in many cases).
Most damage-based spells increase damage OR target extra creatures, but which one is fixed per a given spell. With Scorching Ray, every upcast level produces an additional ray, giving you the flexibility at cast time to either spread out targets or pour more damage into one.
With a warlock, this may not be super interesting since Eldritch blast scales the same way with character level.
So I have been playing a bard recently and it turns out that they don't do much blasting, which means I started to be more creative with my spell choices and upcasts.
Aid
There are bunch of other buffs that are concentration based but by targeting the entire party on upcast make parts of the game that are often dangerous or difficult trivial.
Enhance Ability
I recently started really enjoying upcasting this spell in non-combat situations. Do you need your party of dummies to suddenly get better at a given thing? Just upcast this and give them advantage for the duration.
I play in a party with three charisma casters (including me) and this became a pretty strong combat buff. It turns out that you can counterspell pretty much anything when you get advantage on the ability check.
I don't think it is clear if you can choose different effects for different targets when upcasting the spell (question that addresses it is here) but if yes, it becomes even better.
Fly, Invisibility
Armor of Agathys
"If you are lucky, you will do more damage" is true of any spell with damage dice. But upcast spells typically have multiple damage dice, and as a result of the law of averages the more dice you have, the more likely your damage will be close to the mean.
But, RAW, magic missile has only a single damage die; you roll just once and apply the roll to every missile. And it is a small die, just a d4. That means that every time you cast the spell, you have a 25% chance of achieving maximum damage, regardless of the number of missiles. Without a feature that grants it (such as overchannel), the chance of maximum damage of other spells becomes vanishingly small the more dice you add with upcasting. Thus upcasting a typical damage spell is more likely to yield a total closer to the average, which is not true of magic missile.
We can compare magic missile with other spells chosen specifically for their high output of damage per level. We should also remember that very few things resist force damage, and that magic missile damage scales well since it does not require an attack roll and does not permit a save.
The results show that for a single target, magic missile is competitive with the best damage spells of each level (inflict wounds, fireball, blight, and disintegrate). Just as higher level spells begin to outstrip it in damage, the empowered evocation ability, if you have access to it, provides significant increases in damage. If for some reason you are limited in your spell choice, magic missile is a robust selection, given that it is among the most effective spells at slots from first to eighth level.
Magic missile cast with a first level slot gets you 3 missiles, with an average damage of 10.5, but a 25% chance of the maximum damage of 15. In comparison, inflict wounds cast with a first level slot has an average damage of 16.5, but a 25% chance of damage around 23.25 (assuming you are in the upper quartile of damage, with results weighted proportionally by the chance of each roll). With an estimated 2/3rds chance of hitting your target, you could expect a lucky inflict wounds to do 15.5 damage.
'Lucky' magic missile: 15
'Lucky' inflict wounds: 15.5
Magic missile cast with a second level slot gets you 4 missiles, with an average damage of 14, but a 25% chance of the maximum damage of 20. Inflict wounds cast with a second level slot has an average damage of 22, but a 25% chance of damage around 29. With a chance of hitting your target, you could expect a lucky inflict wounds to do 19.3 damage.
'Lucky' inflict wounds: 19.3
'Lucky' magic missile: 20
Magic missile cast with a third level slot gets you 5 missiles, with an average damage of 17.5, but a 25% chance of the maximum damage of 25. Fireball cast with a third level slot has an average damage of 28, but a 25% chance of damage around 341. Taking 83% of the damage to account for the chance of the target making its save, you could expect a lucky fireball to do 28.48 damage.
'Lucky' magic missile: 25
'Lucky' fireball: 28.5
Magic missile cast with a fourth level slot gets you 6 missiles, with an average damage of 21, but a 25% chance of the maximum damage of 30. Blight cast with a fourth level slot has an average damage of 36, but a 25% chance of damage around 441. Taking account for the chance of the target making its save, you could expect a lucky blight to do 36.8 damage.
'Lucky' magic missile: 30
'Lucky' blight: 36.8
Magic missile cast with a fifth level slot gets you 7 missiles, with an average damage of 24.5, but a 25% chance of the maximum damage of 35. If you are casting 5th level slots, you might be 10th level or higher, and if you are an evoker, you can use your empowered evocation to add +5 to each missile (assuming 20 Int) for a further bonus of 35 damage and a maximum of 70. From here on up, you don't really have to rely on the 25% chance of maximum damage damage, since the contribution of the empowered evocation is so significant. Blight cast with a fifth level slot has an average damage of 40.5, but a 25% chance of damage around 49.51. Taking 83% of the damage to account for the chance of the target making its save, you could expect a lucky blight to do 41 damage.
Average magic missile: 24.5
Average blight: 33.6
'Lucky' magic missile: 35
'Lucky' blight: 41
Average empowered magic missile: 59.5
'Lucky' empowered magical missile: 70
Magic missile cast with a seventh level slot gets you 9 missiles, with an average damage of 31.5, but a 25% chance of the maximum damage of 45 and a maximum empowered damage of 90. Disintegrate cast with a seventh level slot has an average damage of 87.5, but a 25% chance of damage around 93 1.
Average magic missile: 31.5
'Lucky' magic missile: 45
Average empowered magic missile: 76.5
Average disintegrate: 87.5 with failed save (no damage if save made)
'Lucky' empowered magical missile: 90
'Lucky' disintegrate: 93 with failed save (no damage if save made)
Magic missile cast with a eighth level slot gets you 10 missiles, with an average damage of 35, but a 25% chance of the maximum damage of 50 and a maximum empowered damage of 100. Disintegrate cast with a eighth level slot has an average damage of 96, but a 25% chance of damage around 105 1.
Average magic missile: 35
'Lucky' magic missile: 50
Average empowered magic missile: 85
Average disintegrate: 96 with failed save (no damage if save made)
'Lucky' empowered magical missile: 100
'Lucky' disintegrate: 105 with failed save (no damage if save made)
At a ninth level slot Meteor swarm, with an average damage of 140, and a 25% chance of damage around 154, is pretty much untouchable by other single-round damage spells.
For a fifth level slot, the 4d8 force damage of a Clenched Fist to a single target is less than the single-round damage of every 5th level Area of Effect spell. Clearly it is not a good choice, especially for a typical three round fight. But when upcast, it gains 2d8 damage for each spell slot, which is good scaling. In a prolonged, tactical fight that might see it on the board for its full duration, the total possible damage is significant. Other damaging spells typically grant saves, which means something like a target's magic resistance dramatically drops expected damage, whereas the Clenched Fist need only hit. Likewise force damage is one of the least-resisted damage types. After the initial cast, it uses a bonus action to strike, which means a non-Concentration caster like an evoker can use it to supplement whatever their main cast is each turn.
Upcast with 6th level slot, it hits for 6d8 (average 27). Assuming two-thirds of attacks hit (but not adjusting for the significant increase of damage on a crit), that is 180 over ten rounds (or 213 with enhanced evocation) with a single 6th level slot.
With a 7th level slot, 8d8+5 is 273 over ten rounds of attack rolls.
With an 8th level slot, 10d8+5 is 333 over ten rounds of attack rolls.
With a 9th level slot, 12d8+5 is 393 over ten rounds of attack rolls.
When I DM'd a 20th level party's climactic campaign-ending fight against Tiamat, the evoker initially wondered how he was going to contribute, given Her high saves, magic resistance, legendary resistance, and elemental immunities. Eventually he chose a strategy with arcane hand as part of both defense (sheltering behind a 7th level wall of force and assailing the Mother of All Dragons with a highly upcast Clenched Fist) and offense (attacking in melee with Tenser's Transformation while throwing in an extra 59hp Clenched Fist punch each round as a bonus action).
1 Upper quartile damage estimate should be used for comparative purposes only. Anydice doesn't display probabilities of less than 1/100th of a percent, and I didn't take the time to calculate these out exactly for the increasingly long tails of the high dice spells.
Jim's Magic Missile + Spirit Shroud
Both spells upcast, and upcasting spirit shroud with your bonus action can be followed by getting within ten feet of an enemy and using an eldritch blast, which will multiply its output- eventually by 4. You can hold the shroud for a minute and repeat this without consuming more spell slots, but also, next round-
You can upcast a Jim's Magic Missile (not a warlock spell) for (spell level +2) attack rolls to multiply the shroud damage. By the time you get to 7th level slots you're looking at 18d4+27d8 damage if everything hits, with anti-heal and -10 speed on as many creatures as you can hit within 10 feet of you; or just one high threat nova target (288 potential damage before crits, and not factoring in the 3d10+9d8 potential from round one, which brings it up to 390 over 12 attack rolls, using two actions, a bonus action, and two slots).
Obligatory mention of Halflings and/or the Lucky feat to save you from rebounds here.
That depends on whether it's Warlock specific or not, the question is confusingly phrased.
For other casters, lots of good ones have been covered in other questions, but for Warlock, because they only get up to level 5 spell slots(the Mystic Arcanum spells are once/long rest class features, they don't give higher level spell slots), Armor of Agathys is by far the best upcast:
A protective magical force surrounds you, manifesting as a spectral frost that covers you and your gear. You gain 5 temporary hit points for the duration. If a creature hits you with a melee attack while you have these hit points, the creature takes 5 cold damage.
At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, both the temporary hit points and the cold damage increase by 5 for each slot.
It's really useless at lvl1, the temp HP can take 1 hit and deal 5 damage, but at level 5, it can absorb 25HP of dmg and deal 50-100 damage, depending on how many hits it took to break it and is a must-have spell to always have known.
