What the game says, what really changes when you play, and the code to unlock it. Steam says 14.6% of players have it.

Artifact of Vengeance
Game text: «Your relentless doppelganger will invade every 10 minutes.»
Every 10 minutes a copy of you (a doppelganger) appears with your survivor and your items, and comes for you. Kill it and it drops an item.
The more items you have, the worse your copy is: with Brainstalks or Focus Crystals it hits very hard. Kill it before it hits: Captain and Railgunner have it easy from range.
It is in the base game. 14.6% of all Risk of Rain 2 players on Steam have it, from Steam's public stats on 31 October 2026.
Read top to bottom, as shown here: three rows of three shapes. Each slot on the generator changes shape when pressed, in this order: square ■, circle ●, triangle ▲, diamond ♦.
In co-op, the group can enable any artifact that any player has unlocked. Artifacts are chosen in the menu before the run starts.
Enter its code (♦■■ / ♦●■ / ♦■■) in the Compound Generator on Sky Meadow or Helminth Hatchery, use the laptop and beat the Bulwark's Ambry trial: take the key and break the reliquary.
Every 10 minutes a copy of you (a doppelganger) appears with your survivor and your items, and comes for you. Kill it and it drops an item. The more items you have, the worse your copy is: with Brainstalks or Focus Crystals it hits very hard. Kill it before it hits: Captain and Railgunner have it easy from range.
14.6% of all Risk of Rain 2 players on Steam have it, from Steam's public stats on 31 October 2026.
Sources: the game's English text (artifact name and description), the Risk of Rain 2 wiki (riskofrain2.wiki.gg: the Artifacts page and this artifact's page, read on 31 October 2026; code, Compound Generator and Bulwark's Ambry) and public Steam stats. The plain-words text is ours. Icons: Risk of Rain 2 (Hopoo Games / Gearbox).