Race

Undead

Plague · levels at 3 / 6 / 9 different pieces

Hits every non-Undead piece on both sides, including your own, except pieces holding Ascetic's Cap. Only base HP is reduced.

3
All non-Undead pieces on the board lose 10% max HP.
6
They lose another 30% (-40% total).
9
Plagued pieces also lose 5% max HP every 2s, down to a minimum of 5% of their base max HP.
Note: The tooltip says every 2.5 seconds; the game uses every 2 seconds.

Official in-game tooltip

Species Trait: Plague
Active when you have at least (N) different Undead pieces on board. (Multiple effects stack)

(3) Undeads: All non-undead pieces of both sides in the battle gain the plague effect: maximum health -10%.
(6) Undeads: All non-undead pieces of both sides in the battle gain the plague effect: maximum health -30%.
(9) Undeads: All plague-infected pieces lose 5% max health per 2.5 secs (no less than 5%).

Many humans had unfortunately lost their life to the Scourge King during his conquest of his homeland. The victims were raised as Undeads with cruelty, turned into minions of the fleshless legion, and were forced to destroy their own kingdom under the command of the Fallen Prince.