Choker of the Duelist

Wondrous item, rare (requires attunement)

This choker is worn by those who focus intently on finishing a single foe at the expense of all else. The longer you engage a single target, your vision begins to tunnel and focus on your mark. When you hit a target with a melee attack, gain the First Strike effect. Every successive hit adds the next effect additively. If you do anything on your turn besides attacking that target or moving, you lose all effects.

• First Strike. Gain advantage on attacks against the target. All creatures besides the target have advantage on attacks against you.
• Second Strike. Add +5 to damage rolls against the target. Your AC counts as 2 lower for all attackers except your target.
• Third Strike. Your melee attacks critically hit on a roll of 18-20 against the target. You gain vulnerability to non-magical damage dealt from anyone except your target.