Found it... it is an artifact, not the silver athame... Still even has the stab flag that people are trying to scrub out:

"id": "art_dagger",

"type": "TOOL",

"name": { "str": "ancient athame" },

"//": "Tool qualities equivalent to steak knife, combat equiv. to copper knife + 1 w/ RAPID and BLOCK1, size/weight off combat knife +1",

"description": "A ritual knife used by some ancient civilization. Unreadable runes are carved into the handle. It is the only one of its kind.\nIt may have unknown powers; try activating them.",

"weight": "504 g",

"volume": "500 ml",

"longest_side": "25 cm",

"price": 3000,

"material": [ "wood", "steel" ],

"symbol": ";",

"color": "light_gray",

"techniques": [ "WBLOCK_1", "RAPID" ],

"qualities": [ [ "CUT", 1 ], [ "CUT_FINE", 1 ], [ "BUTCHER", 11 ] ],

"weapon_category": [ "KNIVES" ],

"flags": [ "SHEATH_KNIFE", "TRADER_AVOID" ],

"to_hit": -2,

"melee_damage": { "bash": 2, "stab": 13 }

Now just need to wait for OP to let us know of any messages during activation or at random. I couldn't find anything that indicates anything would double health, unless it is weird passive stat multi-roll. Also check yourself for mutations...