Nine Lives Stealer
Weapon (any sword), very rare, requires attunement · 2014 rules, SRD 5.1
+2 attack and damage; 1d8+1 charges. On a critical hit vs a creature with under 100 HP, it must make a DC 15 Con save or be slain (spends a charge).
Also in the 2024 rules (SRD 5.2).