Major Threat Weapons Legacy (5e Variant Rule)
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Major Threat Weapons
The idea behind major threat weaponry is that if the user has proficiency with the weapon, they automatically hit any target below a certain AC without making an attack roll. This is done by the formula of a number + the user's weapon attack modifier. The number is dependent on the weapon, see the below examples, but shouldn't be higher than 10, and is in average cases 7 or 8.
Normally used against the party, these weapons should be feared by them and the first (or second, depending on the circumstances because healers are sometimes VERY annoying) target to focus on if you are implementing them correctly (With reference to how they're built to be used). As a rule of thumb, any time such a weapon is in play, its attacks require a Dexterity saving throw (DC depends either on the weapon or the caster's spell save DC, depending on the type of weapon), and on a success, they take half damage. However, any automatic hit because of a low AC can also count as a critical, since an attack roll is never made, though this should have a cooldown (I default the cooldown to 10 rounds in the examples). This means that even on a save they take what they would have taken if they got hit normally instead of automatically when this occurs. The damage from them also affects the user's hit points in a way that can't be healed by mundane means, requiring magical or potion-based healing to be effective. You see the damage here? Some Major Threat Weapons will be listed below, including a few modern age guns, special wands, compound bows, etc., to give an idea what I'm going on about. Generally, using this ruleset, any modern and futuristic weaponry would count as major threat weaponry, except maybe modern blades and the like. Technically though, any given weapon could have a Major Threat Weapon version, though it would likely be the highest caliber version of any given weapon type, masterfully crafted (And if it's magic, with only the highest tier magical prowess), and probably have a rarity as if it was a wondrous item.
Here are some examples of Major Threat Weapons, but keep in mind that while these in particular are fairly powerful damage-wise, a Major Threat Weapon is based on its ability to hit without making a roll at a certain AC threshold, NOT in its inherent power. An otherwise perfectly normal sword can be a Major Threat Weapon, if made well enough to cleave through armor without even trying.