Do you want your D&D boss monsters to have a "weak point" that deals extra damage, a la The Legend of Zelda? Here's how:
First, make sure all of your players know that your bosses will have weak points. This entire mechanic is reliant upon the players knowing that they're ABLE to target a creature's weak point.
Alternatively, introduce the mechanic with a boss that is very obviously immune or heavily resistant to damage all over its body except for one OBVIOUS weak point. "Demonic energy swirls all over the abyssal creature, except for the two twitching eyes on the palms of its hands."
Now that your players are on the lookout for weak points, you can make them less obvious. A character can use the Search action in combat to make a Wis (Perception) check with a DC you set, discovering the weak point on a success. A failure lets you tell them a false weakness.
To attack a specific weak point, a character can choose to take a –5 penalty to hit to strike it directly. On a hit, it deals +10 damage and might inflict an additional negative effect, like also bypassing its damage resist or condition immunity, disadv on next attack, etc.
If the character failed their check and misjudged the weak point, they still suffer –5 to hit, but hitting doesn't have the extra effects. They can also guess at the monster's weakness without taking the Search action; they take a penalty, and gain benefits if they guess right.
I envision this house rule, if employed, applying to any creature that has Legendary Actions/Resistances. Any monster with a weak point can use a legendary action to block that weak point for one turn once it's been hit once, increasing the to-hit penalty from –5 to –10.
Why employ this mechanic? It's one way to make your big monsters to feel more like puzzles with correct answers, rather than bags of hit points that'll die if you poke at their ankles enough.
What do you think? Would you use this house rule in your game? Is it missing anything?
