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True strike says:

You extend your hand and point a finger at a target in range. Your magic grants you a brief insight into the target's defenses. On your next turn, you gain advantage on your first attack roll against the target, provided that this spell hasn't ended.

It's a concentration spell with a duration of 1 round.

Imagine I cast true strike in the first round of an encounter. Assume I have 20 in my spellcasting ability.

What spell should I use to achieve the most damage on average in one round with a spell attack roll so it benefits from true strike?

Any PC class is allowed. The focus is on spells and synergy with true strike, so ignore magic items and limited resources (like a sorcerer's metamagic) but you can use constant features (like the draconic bloodline sorcerer's Elemental Affinity).

The target does not have any resistances, vulnerabilities or immunities. The creature type is generic. There should be no setup, but we can have advantage on our first attack roll. The base hit chance is 60%.

(My best finding is inflict wounds in a 9th level slot for 50.82 damage:
11d10 * 0.6 = 60.5 * 0.6 = 50.82)

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I'm pretty sure that inflict wounds is your best bet. The 5e spell list is small enough that it's possible to scan the whole thing, and there's nothing that deals more damage under the specific constraints you listed (pre-cast true strike, attack roll, not using any daily powers other than the ninth-level spell slot).

I've got some space left in this answer, so I'd like to address a related question. This is a question you haven't asked, but it's a question which some of the people reading this answer might be curious about: "I'm fighting something that has Legendary Resistance and will choose to succeed at any saving throws I offer. I have a buff spell pre-cast. What's the most damage I can deal, ideally without walking into melee range?"

It turns out that most of the good answers don't take advantage of the true strike. For instance, the old standby meteor swarm deals half of 40d6 on even a successful save, so that's 70 damage plus whatever bonuses you can get from modifiers.

Perhaps our character doesn't start out at level twenty; perhaps, rather than meteor swarm, we'd like a spell that works great using a ninth-level spell slot but also scales down gracefully. Rules As Written, it's worth looking at magic missile: there's an official, though unpopular, ruling that magic missile multiplies your bonus spell damage per missile. Combining this with Empowered Evocation, Elemental Affinity, and/or Radiant Soul can lead to very large numbers which are technically legal -- although, in practice, your DM is unlikely to permit this.

The true strike spell grants advantage on the first attack per round, but a twentieth-level caster might be willing to use a stronger buff, such as greater invisibility. Used against a creature that can't see through it, this spell grants advantage on all attacks in a given round. With this buff, scorching ray is better than inflict wounds: it fires ten rays maximum and deals 2d6 damage per ray, for an average of 70 damage, or 58.8 damage after applying the 84% hit chance.

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  • \$\begingroup\$ There real question is if True Strike is any good. You answer it nicely. \$\endgroup\$ – András Jul 19 '18 at 8:08
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Crown of Stars (XGtE p152-153) has better total damage output than Inflict Wounds, but it takes multiple actions and you'd have to re-cast True Strike between firing each mote, so there's an opportunity cost in having fewer overall actions. It doesn't do better damage than Inflict on any given hit, but the overall effect is superior. (Similar to Scorching Ray, this would benefit from using Greater Invisibility or some other avenue of gaining advantage besides True Strike.)

But that said, even a guaranteed hit with an Inflict spell doesn't stand up to just using one of the upper tier spells. If raw damage output is your goal, and you have 9th level spells available, a Meteor Swarm that the target saves against is still going to deal nearly 50% more damage than hitting with an Inflict. And Power Word Kill doesn't have an attack roll or save, and doesn't deal damage per se, but if the target has between 50 and 100 HP, it's still strictly superior to any Inflict.

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Given the constraints, I think a lvl 20 storm sorcerer with witch bolt can deal more damage.

XGtE pg52:

HEART OF THE STORM: At 6th level, you gain resistance to lightning and thunder damage. In addition, whenever you start casting a spell of 1st level or higher that deals lightning or thunder damage, stormy magic erupts from you. This eruption causes creatures of your choice that you can see within 10 feet of you to take lightning or thunder damage (choose each time this ability activates) equal to half your sorcerer level.

So if the enemy is within 10 feet you deal him 10 lightning damage no matter if you hit or not, and then you deal 9d12 == 58.5 *0.84 = 49.14 (+10). The 0.84 figure is the hit chance for a 60* to-hit with advantage. So you are dealing 59.14 at 10 feet range, instead of 50.82 as a melee hit (and other effects as well, look up the Storm sorcery).

If XGtE isn't a valid source, with a lightning dragon sorcerer you still outdamage inflict wounds with witch bolt as long as you are at 16 charisma or more: (+3 damage for elemental affinity): 9d12+3 == 61.5 *0.84 = 51.66. Of course, more Charisma means you deal more damage.

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    \$\begingroup\$ Witch Bolt requires concentration, so it interrupts and stops True Strike before it could provide the Advantage. Now the hit chance is only 0.6, bringing the damage down to 45.1 \$\endgroup\$ – András Jan 8 '18 at 14:47
  • \$\begingroup\$ That's a good point. I've seen it ruled otherwise (true strike ending as you roll damage, not before you do) but it is a valid point. In fact, I've seen true strike used to grant advantage to witch bolt in an answer you edited, rpg.stackexchange.com/a/51560/40470 . I've started a question here to see if there's any definitive ruling in this regard: rpg.stackexchange.com/questions/113035/… \$\endgroup\$ – LordHieros Jan 8 '18 at 18:07
  • \$\begingroup\$ XGtE on page 5 says "As soon as you start casting a concentration spell, concentration ends on other spells" Ref this question: rpg.stackexchange.com/questions/113052/… \$\endgroup\$ – RonV Jul 18 '18 at 12:36
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Inflict wounds is the best pure spell at 50.82 (one cast with a single spell attack roll).

However, there are a couple options which deal more damage in this situation - mainly options that take advantage of adding an ability modifier to damage.

With a single attack roll:
A Paladin-Warlock could stack both Divine and Eldritch smite on a single cast of Booming Blade (assuming a D8 weapon for ease of calculation) for 14d8+5 damage, or 57.12 damage with advantage. This does use two spell slots, but they're 4th and 5th instead of a single 9th.

With multiple attack rolls (and kibitzing over what constitutes resource expenditure, weighing a 9th level spell slot against 2 sorcery points):
A Warlock/Sorcerer could cast Eldritch Blast followed up by a Quickened Eldritch Blast for 1d10+5 (8.82) at advantage and 7d10+35 (44.1) without advantage for a total average of 52.92

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    \$\begingroup\$ Dragon Sorcerers only add their Charisma to 1 damage roll \$\endgroup\$ – András Jan 8 '18 at 20:47
  • \$\begingroup\$ @András I was actually unaware of that limitation - thanks for the clarification. \$\endgroup\$ – Speedkat Jan 8 '18 at 21:07

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