Today’s Dungeons and Dragons magic item comes from trying to figure out how to get advantage, when playing solo. Here, then, is the Insightful Weapon. Note, it is not like normal magic items in DnD, see the note.
Note: This weapon learns about a creature during an encounter. If you attack an orc, it learns about orcs for later in the encounter. If you later attack a hill giant, the weapon starts over (that is, you have disadvantage on that attack against the hill giant). This weapon is best used when you are fighting the same type of creature in an encounter. It might not be your best choice if the encounter is made up of several types of creatures.
Insightful Weapon
Wondrous Item, Rare, Attunement
An insightful weapon learns the weaknesses of your enemies. The longer you are in a fight, the better you get at hurting your enemy. In this instance, an enemy is one kind of creature (orc, or elf, or beholder, etc.). Nearly all insightful weapons are melee weapons.
In the first round of an encounter, you have disadvantage on attack rolls, but +2 to your Armor Class (AC), as the weapon is testing your enemy for weaknesses. In the second round, you attack normally, and your AC bonus is +1 (assuming you attack the same type of creature as in round 1). In subsequent rounds, as long as you attack the same type of creature as you have at least twice this encounter, you have advantage on all attacks, but your AC now has a -2 penalty. Any time you change the type of creature you attack, your weapon starts learning about that creature as if it is the start of the encounter (for that creature type).
Here is an example:
In round 1, you attack a kobold. You have disadvantage, but your AC is +2. In round 2, you attack a kobold. Your AC bonus is +1, and you attack normally. In round 3, you attack a dragon. You have disadvantage, but your AC is +2. In round 4, you attack the dragon again. Your attack normally, and your AC bonus is +1. In round 5, you attack the dragon with advantage, but your AC is 2 lower. In round 6, you attack a kobold. You have advantage, but your AC is 2 lower. In round 7, well, all your enemies are dead or have fled. The encounter is over. Your weapon resets, and when you next see a kobold, you have disadvantage, but +2 to your AC, etc.
Don’t forget to check out my items on DMs Guild!