From Stardew Valley Wiki
|It's glowing with a mysterious energy.|
|Source:||Monster drop below Level 60 in The Mines|
|Critical Strike Chance:||.05|
|Stats:||Crit. Power (+25)|
|Purchase Price:||Not Sold|
Striking a monster with a Dark Sword gives approximately a .08 chance of increasing the player's Health, if it is below max.
It's possible that the Dark Sword was never implemented in the game, and is unobtainable.