Space Marine 2 Chaos Siege Mode: How It Works in 2026

Chaos Siege is the free Space Marine 2 mode, added on 1 October 2026 with Patch 15, that lets you play Heretic Astartes in wave-based PvE for the first time, holding positions in an Avarax hive city against Tyranids, Cadian troops and loyalist Space Marines. It builds on the original Siege mode from June 2025, so the core loop of waves, a resource shop, bosses every fifth wave and an endless phase after wave 15 still applies. This guide covers what is new in the Chaos version, how the shared Siege systems work as of November 2026, what has been rebalanced since launch and how to earn the weekly rewards.

We checked this against the official Patch Notes 8.0, 13.0 and 15.0, Hotfixes 15.1 and 15.2, the September and October 2026 Community Updates on Steam and the developers' September 2026 Q&A recap, in November 2026.

What Chaos Siege Mode Adds

Until Patch 15, the Chaos Space Marines were only playable in Eternal War PvP. The September 2026 Community Update framed the new mode as a chance to see the battlefield from the other side: Heretic Astartes fighting for control of Avarax, culling the Tyranid swarm and facing Imperial forces and Space Marines who try to stop them from conquering the hive city.

Here is what the official notes confirm about the mode.

• You play as Heretic Astartes, supported by Cultists and even a Helbrute.
• Enemies come from two sides: the Tyranid Hive Fleet and the Loyalists, including Cadian forces and loyalist Space Marines.
• The new Siege map is called Hive. Somewhere in the heart of the hive city, Heretic Astartes defend strategically vital locations and try to perform blasphemous rituals.
• Two sectors were available at launch, and the developers said a third is in development.
• The mode is free for all players, released alongside paid Season Pass 3 cosmetics that are not required to play it.

The developers have not published every detail of the Chaos roster or reward track in the notes we checked, so treat third-party wave-by-wave breakdowns with caution until the official sources catch up. What is clear is that the mode reuses the Siege framework, which has changed a lot since it first launched.

The loyalist Siege has not gone anywhere, so you now have two flavors to choose from. The original sets your Space Marines against Tyranid and Thousand Sons waves inside an Imperial fortress on Kadaku, with Cadians and a Dreadnought as your backup. The Chaos version flips the perspective: you are the invaders inside an Avarax hive city, and the enemy list includes Imperial Guard and loyalist Astartes who use many of the same tools you rely on in the other mode. If you are new to Siege in general, the loyalist version is the gentler place to learn the shop and wave rhythm, because its enemy mix has been tuned and documented for longer.

How Siege Works Under the Hood

The original Siege, added in Patch 8.0 on 26 June 2025, set the rules that Chaos Siege builds on. It is a three-player PvE mode about holding a position against endless waves.

• Waves are grouped by sector. In the first map, each of three sectors held five waves before the squad moved on, eventually cycling back to earlier positions.
• Every fifth wave brings a boss encounter, sometimes several elite enemies at once, and beating it moves you forward.
• Short one-minute intermissions between waves let you resupply.
• Side objectives appear during combat and pay extra resources.
• You earn Siege resources from combat and objectives and spend them in a shop on ammunition, Medicae Stimms, equipment, armour boosts and Guardian Relics, which behave exactly as in Operations.
• Reinforcements are part of the shop too: each player can have one summoned specialist Space Marine, Cadian troopers add firepower, and the squad can pool resources for a Dreadnought.
• After wave 15 the mode becomes endless, with tougher enemies, fewer resources and shorter preparation times.
• Extracting before you fall earns Requisition Points, Armoury Data and XP.

In the Chaos version, the reinforcement roles shift to the traitor side: the official notes name Cultists and a Helbrute as your support. Hotfix 15.1 refers to a summonable Helbrute, which fits the same idea of buying help mid-run.

Blessings, Atonements and the Timer

Patch 13.0 on 21 May 2026 overhauled Siege as a whole, and those rules carry into the Chaos mode. The biggest addition is the Blessings and Atonements system: every five waves you receive a permanent Blessing at a Shrine and endure shifting Atonements, which push each run toward a different build.

The April 2026 Community Update explained the choice in more detail: you pick one Blessing out of three every five waves, one Atonement is applied every five waves as well, and you can reroll the Atonement by spending Siege currency. Other Siege 2.0 changes still shape every run.

• The timer no longer ends the game. When it runs out, enemies get much stronger each time it triggers, so slow play is punished rather than instantly failed.
• The difficulty curve is steeper, with enemies growing stronger much quicker and earlier, to make sessions shorter and more intense.
• The Hive Tyrant and the Vortex Beast joined the boss pool, spawning on waves such as 5, 10 and 15.
• The shop gained a Tarantula Sentry Gun and its upgrade, a Tech-priest, a Minefield, Disciplined Reinforcements, a Techmarine and Explosive Resupply.
• Two new random side objectives were added.

Patch 15.0 temporarily removed the Cursed Relic from the Atonement pool, and the developers said Patch 16 will take another look at Blessings and Atonements in Siege. Since Patch 14.0, the Thousand Sons Scarab Occult Terminator Sorcerer can also appear throughout Siege, so expect a Terminus-level caster among the threats.

Balance Changes Since Launch

Chaos Siege received fixes quickly. Hotfix 15.1 on 8 October 2026 and Hotfix 15.2 on 20 October 2026 both touched the mode.

• Players could take damage from friendly Chaos portals; that was fixed.
• The summonable Helbrute's melee damage was cut by 30 percent.
• Several weapons were not using Chaos skins in Chaos Siege; fixed.
• Loyalist Marines became much more effective at killing Cultists, which makes protecting your support troops more important.
• An enemy Loyalist Marine could spawn behind closed gates in some cases; fixed in 15.2.
• A softlock where Space Marines could get stuck in a transition zone after a wave sequence in Siege was fixed in 15.2.

The October Community Update summed the month up as various balance changes and bug fixes in Chaos Siege. In the September Q&A, the Game Director also mentioned a future pass on how Chaos performs against Tyranids, and said loyalist customisation remains the priority but that Chaos cosmetics are not ruled out. Expect more tuning; the Year 3 roadmap lays out what Patch 16 and beyond should bring.

Rewards: Accolades, XP and Events

Siege is one of the main weekly sources of Accolades. Since Patch 13.0, reaching wave 15 on Normal once per week pays 50 Accolades and reaching wave 15 on Hard pays 100, and Patch 14.0 confirmed the reward is granted on reaching wave 15 itself. Our guide to earning Accolades shows how Siege fits with Stratagems and PvP in a weekly plan, and the Heroic weapons guide covers what to spend them on.

Events add more. The Chaos on Avarax Community Event, which ran until 3 November 2026, paid 100 Accolades for surviving a combined total of 15 waves in Chaos Siege across any number of sessions, on any difficulty. Patch 15 also launched Trials of Resolve #3, a set of hard Ordeals that includes Siege challenges. Finally, completing Siege still pays standard XP, Requisition and Armoury Data when you extract, so a long run feeds class progression too.

Tips for Your First Chaos Siege Runs

The loyalist Siege habits carry over, with a few Chaos-specific twists.

1. Start on Normal. Hard Siege is end-game content, and the Game Director said in October 2026 that a level lock is planned for it. Our difficulty levels guide explains how Siege's two settings compare with Operations tiers.
2. Spend between waves, not during them. Intermissions are short, so agree on a shopping plan as a squad before the next wave starts.
3. Protect your Cultists. After Hotfix 15.1, enemy Loyalist Marines kill Cultists much faster, so keep your support away from loyalist spawn lanes.
4. Treat the Helbrute as a tool, not a carry. Its melee damage was reduced after launch, so pair it with your own focus fire on bosses.
5. Pick Blessings for your squad, not just yourself. A Blessing that boosts the whole team's survival usually beats a personal damage bonus by the time endless waves arrive.
6. Know when to extract. Your rewards come from extracting, so a planned exit after the weekly wave 15 target is better than a wipe at wave 22.

Ready for Siege Without the Grind

Siege rewards a squad with leveled classes and good gear. The store's Space Marine 2 boosting and unlock services cover PvE, PvP, cosmetics and progression for PC, PlayStation and Xbox, if you would rather start Chaos Siege with your classes and arsenal already built up.

Frequently Asked Questions