D&D 5E Mirror Image Math

[Worrgrendel]

I guess your DM doesn't use dispel magic alot.

This PC could go from hero to zero very quickly.

Hit with dispel magic AC goes from potentially 28 to 23 with Shield. Nothing to sneeze at at level 9. Plus there is always Counterspell if reaction still available. As soon as +1 Studded Leather is found then the Mage Armor issue is negated and AC only drops to 26 with Shield. I don't see Dispel Magic as a Hero to Zero issue.

 

[Stoutstien]

Hit with dispel magic AC goes from potentially 28 to 23 with Shield. Nothing to sneeze at at level 9. Plus there is always Counterspell if reaction still available. As soon as +1 Studded Leather is found then the Mage Armor issue is negated and AC only drops to 26 with Shield. I don't see Dispel Magic as a Hero to Zero issue.

They would be stripped of shield, mage armor, and haste on top of being stunned due to haste dropping which would also kill bladesong.
can't shield and CS same round so if they are the only one with CS and they pop shield it's pretty bad.
Dispel magic has a much longer range than CS which is something that gets overlooked alot.

 

[Lucas Blackstone]

They would be stripped of shield, mage armor, and haste on top of being stunned due to haste dropping which would also kill bladesong.
can't shield and CS same round so if they are the only one with CS and they pop shield it's pretty bad.
Dispel magic has a much longer range than CS which is something that gets overlooked alot.

Maybe I am looking at an older version of the Bladesinging tradition but what I read says it ends if you are incapacitated, don medium/heavy armor or shield, or use 2 hands to make an attack. When haste ends you can't move or take actions until after your next turn. I don't see a conflict with bladesong staying up and haste ending.

Losing 10 ac though is still brutal, I agree. I find it amusing to drop dispel magic on a blade singer and watch them freak out if they are hip deep in enemies who get a turn to try to beat them down without crazy ac.

 

[Stoutstien]

Maybe I am looking at an older version of the Bladesinging tradition but what I read says it ends if you are incapacitated, don medium/heavy armor or shield, or use 2 hands to make an attack. When haste ends you can't move or take actions until after your next turn. I don't see a conflict with bladesong staying up and haste ending.

Losing 10 ac though is still brutal, I agree. I find it amusing to drop dispel magic on a blade singer and watch them freak out if they are hip deep in enemies who get a turn to try to beat them down without crazy ac.

No I was wrong actually. I was remembering haste wrong so blade song would stay up. Not as bad as I first thought. Still would be a big blow to the bladesinger's defense and action economy.

 

[Worrgrendel]

They would be stripped of shield, mage armor, and haste on top of being stunned due to haste dropping which would also kill bladesong.
can't shield and CS same round so if they are the only one with CS and they pop shield it's pretty bad.
Dispel magic has a much longer range than CS which is something that gets overlooked alot.

Lucas already pointed out that Bladesong would still be up so we can skip over that.

Most fights do not involve casters. I realize that can also be campaign dependent. To further that, not all caster mobs have Dispel Magic. Additionally, its not hard to figure out whether or not a mob is a caster and my wife is smart enough to figure out if there is a caster on the table then that reaction is held for Counterspell and not Shield. Without Shield available her AC is still good and can weather most attacks, even if she is surrounded and especially if Mirror Image is up. As to the range of Dispel Magic vs Counterspell, how many fights open up at 120 feet? or 90 feet? If the DM wants his caster to spam Dispel Magic at the Bladesinger I feel that to be a waste of the caster's actions while the 5 other party members are completely ignored and burn him/her to the ground.

YMMV

 

[Todd Roybark]

how many fights open up at 120 feet?

If one is fighting Drow, all of them.

 

[Worrgrendel]

If one is fighting Drow, all of them.

So you fight nothing but Drow in all of your campaigns and every encounter in the Underdark is a room that is larger than 120 feet across and they all automatically know to target Dispel Magic on one exact member of the group? That seems a bit niche. There are a plethora of good adventures out there by WOTC and 3rd party DM's Guild that do not take place solely in the Underdark. I recommend a change of pace for your gaming group and have them "step outside" for your next campaign for a little variety. Maybe something with Dragons or a Death Curse or a Vampire lord or even Hell?

 

[Todd Roybark]

LOL, and for you @Worrgrendel, I recommend some encounters with opponents using Longbows at 600’ range.

I presume you are being humorous with a bit of snark, but many creatures have some long range ability. Beholderkin 👁 being a prime example.

 

[Worrgrendel]

LOL, and for you @Worrgrendel, I recommend some encounters with opponents using Longbow’s at 600’ range.

I presume you are being humorous with a bit of snark, but many creatures have some long range ability. Beholderkin 👁 being a prime example.

Yes, humorous with snark. Text doesn’t always convey tone well.

 

[Esker]

I think it might be helpful to work backwards...

Let C(n,m) be your expected number of hits with n attacks to come if you have m images remaining. Then (assuming for simplicity that we pre-cast MI) we are interested in C(n,3) as compared to C(n,0).

Let T(m) be the chance that an image is targeted when you have m of them, and let x and y be as you defined them in the OP. We have T(0) = 0, T(1) = 10/20, T(2) = 13/20, T(3) = 15/20.

Then

C(n,0) = nx

C(1,m) = (1 - T(m)) * x

and for m > 0 and n > 1:

C(n,m) = C(1,m) + (1 - T(m) + T(m)*(1-y)) * C(n-1,m) + T(m) * y * C(n-1,m-1)