5.5E Mirror Image (2024) is an almost guaranteed Crit Buster

 

1. Spell simplified

   As the spell states, you roll a d6 for each duplicate present.

   • 3 Duplicates - 3d6
   • 2 Duplicates - 2d6
   • 1 Duplicates - 1d6

   If any of those rolls is higher than 2, then the duplicate is hit

   What we need to calculate here

   What is the chance that the mirror image will take the hit instead of you?

2. 3 Duplicates

   For 1 Die,
   S = {1, 2, 3, 4, 5, 6}
   P(1, 2) = P(Hits PC) = 1/3
   P(Hits PC) + P(Hits Duplicate) = 1
   
   For the attack to hit the PC, 1 or 2 has to show up on all 3 dies = P(Hits PC) AND P(Hits PC) AND P(Hits PC)
   AND = x (Product)
   ∴ P(Hits PC) x P(Hits PC) x P(Hits PC)
   => 1/3 x 1/3 x 1/3 = (1/3)^3
   ∴ P(% Hits Duplicate) = 100 - P(% Hits PC) = 100 - ((1/3) ^ 3) x 100 = 96.3%

   Chance for Duplicate to be hit = 96.3%
3. ----------------------------------------

4. 2 Duplicates

   For 1 Die,
   S = {1, 2, 3, 4, 5, 6}
   P(1, 2) = P(Hits PC) = 1/3
   P(Hits PC) + P(Hits Duplicate) = 1
   
   For the attack to hit the PC, 1 or 2 has to show up on both the dies = P(Hits PC) AND P(Hits PC)
   AND = x (Product)
   ∴ P(Hits PC) x P(Hits PC)
   => 1/3 x 1/3 = (1/3)^2
   ∴ P(% Hits Duplicate) = 100 - P(% Hits PC) = 100 - ((1/3) ^ 2) x 100 = 88.8%

   Chance for Duplicate to be hit = 88.8% --------------------------------------

5. 1 Duplicate

   For 1 Die,
   S = {1, 2, 3, 4, 5, 6}
   P(1, 2) = P(Hits PC) = 1/3
   P(Hits PC) + P(Hits Duplicate) = 1
   
   For the attack to hit the PC, 1 or 2 has to show up on the die = P(Hits PC)
   => 1/3 
   ∴ P(% Hits Duplicate) = 100 - P(% Hits PC) = 100 - (1/3) x 100 = 66.6%

   Chance for Duplicate to be hit = 66.6%

6. When to use this?

   When you are already cannot be hit. This is simply ensures crit busting, making you truly impossible to hit. If your AC is so high that only a crit can hit you with a 5% chance to crit normally, and 0.25% chance to crit at disadvantage, this is your chance to be hit

   3 Duplicates

   Without Blur

   5% x 3.7% = 0.185%

   0.185% Chance to hit

   With Blur

   0.25% x 3.7% = 0.00925%

   0.00925% Chance to Hit

   2 Duplicates

   Without Blur

   5% x 11.2% = 0.56%

   0.56% Chance to hit

   With Blur

   0.25% x 11.2% = 0.028%

   0.028% Chance to Hit

   1 Duplicate

   Without Blur

   5% x 33.3% = 1.665%

   1.665% Chance to hit

   With Blur

   0.25% x 33.3% = 0.08325%

   0.08325% Chance to Hit Of course, Blade Ward is a decent concentration as well (S-tier if you are an Eldritch Knight), which reduces enemy hit chance by 12.5% on average. But this is an edge-case Crit scenario, which is the reason i am omitting Blade Ward calculations.

7. When not to use this?

   • When you have low AC in the first place. Multiattack will instantly end it. Although, if it is the ONLY thing you can do to save yourself, go for it.
   • When enemies are blinded or have anything better than Darkvision. NEVER CAST IT IF THIS IS THE CASE!