Games like The Black Siege
Games like The Black Siege include Mage and Monsters, Bad North: Jotunn Edition, FTL: Faster Than Light, Squad, and Thronefall. We matched them by the tags players give The Black Siege on Steam (Auto Battler, Action, Perma Death, PvE, and Roguelike): closest matches first, and among equally close ones the better-known games.

Mage and Monsters
Shared: Auto Battler · Perma Death · Strategy

Bad North: Jotunn Edition
Shared: Auto Battler · Wargame · Roguelike

FTL: Faster Than Light
Shared: Roguelike · Strategy · Perma Death

Squad
Shared: PvE · Military · Action

Thronefall
Shared: Action · Strategy · Minimalist

Arma 3
Shared: Action · Military · Simulation

Shape of Dreams
Shared: PvE · Action · Top-Down

Door Kickers 2: Task Force North
Shared: Action · Top-Down · Strategy

Arma Reforger
Shared: Action · Military · Strategy

Heat Signature
Shared: Action · Roguelike · Top-Down

Nuclear Option
Shared: Action · Military · PvE

Men of War: Assault Squad 2
Shared: Strategy · Action · Military

Cluckmech Oasis
Shared: Roguelike · Strategy · Action

Sir, We Have an Orc Problem
Shared: PvE · Military · Strategy

Totally Accurate Battle Simulator
Shared: Strategy · Simulation · Colorful

The Binding of Isaac
Shared: Roguelike · Action · Top-Down

War Thunder
Shared: Action · Simulation · Military

Bomber Crew
Shared: Strategy · Action · Simulation

Mage and Monsters II
Shared: Auto Battler · Perma Death · Strategy

Call to Arms
Shared: Action · Strategy · Military

Mechabellum
Shared: Auto Battler · Strategy · Wargame

Shieldwall
Shared: Action · Strategy · Colorful

Risk of Rain (2013)
Shared: Action · Roguelike · Perma Death

Hades II
Shared: Action · Roguelike · Indie