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Chaos bolt from UA had the nifty feature of bouncing targets in the event of your 2d8 damage rolling up doubles.

While some people complained about the relatively low damage output, no one highlighted the interesting interplay between chaos bolt and the empowered/twinned spell metamagics, that I saw.

Basically, this would seem to give you a much higher chance of a ricochet, since you can reroll either one of the two d8s on each hit, effectively a 25% chance of ricochet in addition to the higher average damage.

Conversely, Twinned Spell makes two separate attacks, and while each individual attack has a 1/8th chance to ricochet, you're still at a 15/64th's chance of at least one ricochet and already dealing more damage on average with the first attack.

Assuming you can only choose one of these two metamagics, which would yield the most average damage after accounting for ricocheting?

Assuming you could add both metamagics to the spell, what would be your average damage and number of ricochets, assuming you always select the lowest damage die to reroll?

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – doppelgreener Jun 13 '17 at 17:05
  • \$\begingroup\$ Are you asking for academic reasons? Chaos Bolt can't be twinned since it has the potential to target more than one creature. \$\endgroup\$ – Doval Jun 13 '17 at 21:30
  • \$\begingroup\$ @doval pretty sure that's not the appropriate interpretation for the twinned spell metamagic feat as applied to chaos bolt. Might be worth asking a second question though. \$\endgroup\$ – Isaac Kotlicky Jun 13 '17 at 21:46
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    \$\begingroup\$ If the spell can affect another creature in any way, the spell is targeting the creature too. Jeremy Crawford talks at length about this in the Dragon Talk podcast episode on spell targeting. Note also that Chain Lightning doesn't work and it has the same targeting scheme, except not luck-based. \$\endgroup\$ – Doval Jun 13 '17 at 22:43
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    \$\begingroup\$ @Doval a more direct answer than the chain lightning, specifically in response to chaos bolt. Took a bit of searching. He didn't say "no" but he clearly seems to mean that. Still worth the academic exercise... \$\endgroup\$ – Isaac Kotlicky Jun 13 '17 at 23:12
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Computing the ricochet rate is actually simpler than it looks:

  • The chance of 2d8 NOT rolling doubles is (1*7/8)=7/8=87.5%
  • If we get to choose whether to reroll one of the two dice we multiply that AGAIN by the chance of rolling doubles, giving us 7/8*7/8=49/64=~76.6%
  • On a critical hit (5% of the time), you roll a total of 4d8, remembering that ANY double results in a ricochet.
  • The chance of 4d8 NOT rolling doubles is (1*7/8*6/8*5/8)=41%
  • If we get to choose whether to reroll one of the four dice we multiply that AGAIN by the chance of rolling doubles on the fourth die, giving us 41*62.5%= 25.6%
  • The upshot is that a standard chaos bolt will ricochet 12.5% of the time, but an empowered chaos bolt will ricochet 23.4% of the time.
  • On a critical hit, a standard chaos bolt will ricochet 59% of the time, while an empowered chaos bolt will ricochet at a whopping 74.4% rate!

In order to figure out HOW MANY times we'd ricochet on average, we use a version of the multiplier effect. Simply put, our average number of targets hit is 1/(chance of no ricochet).

  • For a regular bolt, that's 1/(7/8)=8/7=~1.14, a twinned spell bolt multiplies this by 2 for 2.28
  • For an empowered bolt with one reroll, that's 1/(49/64)=64/49=~1.31, 2.62 as a twinned spell
  • For a crit from a regular bolt, that's 1/(.41)=2.44 targets, 4.88 as a twinned spell
  • For an empowered crit, our average is 1/(.266)=3.9 targets, 7.8 as a twinned spell.

Assuming every attempted attack hits, your expected number of targets including the 5% chance of a crit is:

  • (1.14*95%+2.44*5%)=1.21 for a regular chaos bolt
  • (1.21*2)=2.42 for a twinned chaos bolt
  • (1.31*95%+3.9*5%)=1.44 for an empowered chaos bolt
  • (1.44*2)=2.88 for a twinned, empowered chaos bolt.

For our first two results, the expected damage is the number of targets multiplied by the average damage roll of 2d8 (which is 9). So a standard bolt deals 10.89 damage while a twinned bolt deals 21.78 on average.

Figuring out the average damage on the empowered bolt is... more complex, since you wouldn't reroll when your initial rolls were doubles, you have to compute the probability of a rerolled die exceeding the original damage die, and recompute the average value for the rerolled die.

For the case of an empowered hit from a chaos bolt, we can divide our calculation into three parts:

  1. The expected value of rolling doubles
  2. The expected value of either the die dropped or the die kept in the event doubles are not rolled
  3. The expected value of the replacement die roll

Note that we don't reroll doubles, nor is there never a point where we don't reroll without them - we are optimizing for ricochet and thus will ALWAYS go for doubles.

  1. This is fairly straightforward, since the probability of rolling doubles is 1/8 and the probability of rolling any particular set of doubles is 1/8th of that (1/64). The sum of (1+1)/64 + ... (8+8)/64 is identical to 2 times the average roll ((1+2+3+4+5+6+7+8)/8) times the probability of rolling doubles (1/8). The result is an EV of 9/8 for the doubles.
  2. This gets a little bit more complicated. We need to compute the probability for each possible number that it will remain when the other die is dropped.
    • There is no chance for us to have a 1 remaining when dropping a die, since that would mean the other die was also a 1 and we would not choose to reroll.
    • For a 2, we could have rolled (2,1) or (1,2) out of 64 options, so we have a 1/32 chance that our highest die before reroll is a 2.
    • The probability formula for having a highest side k on two n sided dice is 2*(k-1)/n^2. Note that summing this formula across all sides and the probability of rolling doubles adds to 1, since you will either have doubles OR the remaining die.
    • The expected value of the die remaining after drop is 5.25
  3. For the reroll, our expected value is the average roll (9/2) times the probability of not rolling doubles (7/8), which is 63/16, about 3.94

Adding all of these together, we get an EV of 10.31 for an empowered Chaos Bolt, a 15% increase in damage. But what about a critical hit? Computing the "lowest die" probability is a significantly more complicated problem that I'm still working on...

Placeholder until I can craft a computational model to explain the last part of the question.

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  • \$\begingroup\$ Writing notes here, since I don't currently have access to the work I was doing earlier. Basically you calculate separately the expected value for crit and non crit doubles (or triples or quadruples for crits), then you calculate the EV for the non-doubles after drop, then add the EV of the reroll die. You need to calc total average damage breaking crit apart from regular hits. Comments on the above methodology appreciated. \$\endgroup\$ – Isaac Kotlicky Jun 13 '17 at 22:38
  • \$\begingroup\$ In working out this answer I've created a spreadsheet that allows for arbitrary computation for doubles probability for up to 20dX and 6 rerolls. If there's interest I can upload to Google docs and post a link in the answer. \$\endgroup\$ – Isaac Kotlicky Jun 14 '17 at 0:21
  • \$\begingroup\$ I would appreciate the link! \$\endgroup\$ – B. S. Morganstein Jun 15 '17 at 14:40
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    \$\begingroup\$ @B.S.Morganstein I'll send the file to my email so I can upload it. It's still a work in progress, so I'd appreciate the extra sets of eyes... \$\endgroup\$ – Isaac Kotlicky Jun 15 '17 at 16:53
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Since a Twinned chaos bolt can't exist,[1],[2] we can set its expected damage output to zero in any calculation.

That means that an Empowered chaos bolt (which can exist) is necessarily more optimal.

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  • \$\begingroup\$ As discussed in the comments on the question, we are aware that this is an academic issue. Regardless, you haven't addressed what the average damage would be... \$\endgroup\$ – Isaac Kotlicky Jun 14 '17 at 21:36
  • \$\begingroup\$ The question doesn't ask that, though? \$\endgroup\$ – SevenSidedDie Jun 14 '17 at 22:19
  • \$\begingroup\$ "Assuming you could add both metamagics to the spell, what would be your average damage and number of ricochets, assuming you always select the lowest damage die to reroll?" \$\endgroup\$ – Isaac Kotlicky Jun 15 '17 at 0:12
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    \$\begingroup\$ @IsaacKotlicky I thought you meant the average damage for just Empowered chaos bolt (which the question doesn't ask). I didn't address the “assuming you could…” part because it can't be done, so it's an obviously false assumption. \$\endgroup\$ – SevenSidedDie Jun 15 '17 at 1:48
  • \$\begingroup\$ someone using the pre errata version of twinned spell would be correct in assuming they could twin chaos bolt. It's the added language that restricts it to "only targets one creature" that precludes its use here, as the spell "targets one creature" at a time. Which was a bald reading of the original feat. \$\endgroup\$ – Isaac Kotlicky Jul 5 '17 at 2:15

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