What the game says, what really changes when you play, and the code to unlock it. Steam says 21.6% of players have it.

Artifact of Swarms
Game text: «Monster spawns are doubled, but monster maximum health is halved.»
Twice as many monsters spawn, each with half their maximum health. Bosses too: two teleporter bosses, two Mithrix.
For more action and more kills (on-kill items, Artifact of Sacrifice). Careful at the end: two Mithrix steal twice the items.
It is in the base game. 21.6% 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.
Twice as many monsters spawn, each with half their maximum health. Bosses too: two teleporter bosses, two Mithrix. For more action and more kills (on-kill items, Artifact of Sacrifice). Careful at the end: two Mithrix steal twice the items.
21.6% 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).