What the game says, what really changes when you play, and the code to unlock it. Steam says 16.9% of players have it.

Artifact of Kin
Game text: «Monsters will be of only one type per stage.»
Each stage spawns only one monster type, picked from those that can appear there. If it picks a boss (golem, titan...), the whole stage is full of bosses and the teleporter spawns hordes of them.
The shortcut for the kill-15-bosses-in-one-run challenge, the one that unlocks Visions of Heresy (one of the four Heretic pieces). With a bad type (wisps, worms) the stage is hell: restart.
It is in the base game. 16.9% 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.
Each stage spawns only one monster type, picked from those that can appear there. If it picks a boss (golem, titan...), the whole stage is full of bosses and the teleporter spawns hordes of them. The shortcut for the kill-15-bosses-in-one-run challenge, the one that unlocks Visions of Heresy (one of the four Heretic pieces). With a bad type (wisps, worms) the stage is hell: restart.
16.9% 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).