I'm always too scared to use Runic Shield. Any ways to fix this?

[FoxWolFrostFire]

Okay. So I love the Rune Knight easily one of my favorite sub classes really fits my niche you know? How ever...I can't shake the feeling that Runic Shield is literally a dead level because I refuse to use this feature because it lacks a safe guard.
I don't want to go "Don't worry wizard I got you!" and watch him get smeared by a crit he wasn't facing until my friendly help offered him. Any advice to get over this? Because sure the same chance he can roll a natural 1 and totally miss. But honestly Natural 20>Natural 1. So like it seems like a bad choice to make that gamble.

[LudicSavant]

Okay. So I love the Rune Knight easily one of my favorite sub classes really fits my niche you know? How ever...I can't shake the feeling that Runic Shield is literally a dead level because I refuse to use this feature because it lacks a safe guard.
I don't want to go "Don't worry wizard I got you!" and watch him get smeared by a crit he wasn't facing until my friendly help offered him. Any advice to get over this? Because sure the same chance he can roll a natural 1 and totally miss. But honestly Natural 20>Natural 1. So like it seems like a bad choice to make that gamble.

Runic Shield is a strong defensive ability because it allows you to reroll the die after a hit is confirmed. If you're not using it, you're leaving a ton of value on the table (exactly how much depends on the shielded person's AC vs the enemy's attack bonus and damage; generally the higher their miss chance and damage threatened on a hit, the better it is to Runic Shield).

If you take a couple new crits while using it, that's completely fine, you shouldn't even bat an eyelash or go 'oh no,' you should be expecting it and comfortable with the possibility of it happening. This is all within the parameters of your plan. Your goal with Runic Shield is to drag down the average total damage taken over the course of the adventure, and the occasional doubled die isn't going to overwhelm that.

Note Runic Shield is generally better spent on people who are harder to hit; you want to maximize your chances of turning a hit into a miss. They shouldn't need a natural 1 in order to miss on the new roll. You also can use Runic Shield to negate enemy crits.

Instead of weighing the chance of a natural 1 vs the chance of a 20, you should be weighing the chance of (any number that is a miss) vs a natural 20. All of those numbers are just as bad for Team Monster as a 1.

[Contrast]

So a hit is (damage die)+stat, while a crit is (damage die*2)+stat, meaning a single crit is not quite as bad as two hits.

So in order to be justifable Runic Shield will need to deflect at least two hits per crit it causes to work out. Lets imagine our enemy had a 15/20 chance to hit our ally (hitting on a 6 on the die). Rerolling would have a 5/20 chance of making them miss and a 1/20 chance of making them crit. For every 5 misses we cause, we'll cause 1 crit. But that's well above our 2 misses 1 crit threshold so we should be using Runic Shield. The harder it is for them to hit, the better the odds look.

Plus of course, if they're already rolling a 20, there's nothing to lose anyway.

Hopefully that math both checks out (edit - it didn't!) and encourages you to get a bit more use out of the feature.

Edit - Actually now that I think about it the math is even better than that given the damage expectation if you do nothing is already a hit so the crit is actually only adding the damage die - less than a single hit. At that point you only need to cause a single miss per crit for the math to work out which is always going to be the case even if the enemy is only missing on a nat 1.

[LudicSavant]

Instead of weighing the chance of a natural 1 vs the chance of a 20, you should be weighing the chance of (any number that is a miss) vs a natural 20. All of those numbers are just as bad for Team Monster as a 1.

Let's take an example. Say a foe does 4d6+10 (24) damage on a hit, has +9 to hit, and rolls a 15 to hit someone with a 20 AC. You then use Runic Shield.

Since they miss on 1-10, there's a 50% chance that they will lose 4d6+10 (24) damage. .5 * 24 = 12 average damage lost.
Since they hit on 11-19, there’s a 45% chance that the damage will remain unchanged.
Since they crit only on a 20, there’s a 5% chance that they will gain 4d6 (14) extra damage. .05 * 14 = 0.7 average damage gained.

So using Runic Shield against that hit is worth +11.3 hp on average.

If you're rerolling a crit, it's especially good. With the same variables, except the enemy rolls a 20 first:

There's a 50% chance that they will lose 8d6+10 (38) damage. .5 * 38 = 19 average damage lost.
There's a 45% chance that they will lose 4d6 (14) damage. .45*14= 6.3 average damage lost.
There's a 5% chance that damage will remain unchanged.

Using Runic Shield in that scenario is thus worth +25.3 hp on average.

Be careful to avoid loss aversion bias. Just because Runic Shield can very occasionally hurt you doesn’t mean that it outweighs the good. Runic Shield is one of the better defensive reactions in the game. Mathematically much better than the “apply disadvantage to a single roll” reactions.

[stoutstien]

Instead of weighing the chance of a natural 1 vs the chance of a 20, you should be weighing the chance of (any number that is a miss) vs a natural 20. All of those numbers are just as bad for Team Monster as a 1.

Let's take an example. Say a foe does 4d6+10 (24) damage on a hit, has +9 to hit, and rolls a 15 to hit someone with a 20 AC. You then use Runic Shield.

Since they miss on 1-10, there's a 50% chance that they will lose 4d6+10 (24) damage. .5 * 24 = 12 average damage lost.
Since they hit on 11-19, there’s a 45% chance that the damage will remain unchanged.
Since they crit only on a 20, there’s a 5% chance that they will gain 4d6 (14) extra damage. .05 * 14 = 0.7 average damage gained.

So using Runic Shield against that hit is worth +11.3 hp on average.

If you're rerolling a crit, it's especially good. With the same variables, except the enemy rolls a 20 first:

There's a 50% chance that they will lose 8d6+10 (38) damage. .5 * 38 = 19 average damage lost.
There's a 45% chance that they will lose 4d6 (14) damage. .45*14= 6.3 average damage lost.
There's a 5% chance that damage will remain unchanged.

Using Runic Shield in that scenario is thus worth +25.3 hp on average.

Be careful to avoid loss aversion bias. Just because Runic Shield can very occasionally hurt you doesn’t mean that it outweighs the good. Runic Shield is one of the better defensive reactions in the game. Mathematically much better than the “apply disadvantage to a single roll” reactions.

Has a solid range as well and the way it worded makes it very nice to use when the attack already had disadvantage or advantage because it rerolls the die that caused the hit rather than the whole shebang.

[Sol0botmate]

I only run a level 10 One-Shot with Rune Knight recently (absolutely LOVE IT!) but I was using Runic Shield a lot.

My general rules of using it were:

1. When enemy crit - I use Runic Shield. In all cases it prevented crit, decrease damage and even made same attack miss totally.
2. When my friend is already hard to hit. So for example if enemies need 15+ rolls to hit him, I will use Runic Shield almost every time if he is surrounded by multiple enemies or fights a high dmg per attack enemy. Of course enemy can score crit, but in most cases worst case scenario will be that they will hit him anyway. But there is much bigger chance that they will miss if they already needed a high roll.
3. I generally didn't use it when enemies already need to roll low to hit low AC friend. If my party member has only 16 AC and enemies hit him on 8+ then using Runic Shield won't help much.
4. If enemy has advantage on attack and both rolls are good (for example one is 15 and other one is 17) I will use Runic Shield as there is bigger chance of him rolling lower than both instead of higher than both.

Using that I prevented a lot of hits, at least 4 critical hits (one from boss that missed instead) and saved my party Druid from dying on enemy turn because I made last hit that would down him miss.

Don't be afraid to use it. Rune Knight has one of the best tools in the game now. Especially Runic Shield and Storm Rune are both one of the best class features in game.