05. The Coming Siege and Possible Fates of Cadia

When the Invasion Begins

The Thirteenth Black Crusade does not need to begin on a fixed date.

Franz should advance the invasion when the campaign has created enough pressure, preparation, and personal investment for the event to matter.

The first unmistakable signs may include:

• Major Warp routes collapsing

• Astropathic silence

• Black Legion fleets leaving the Eye

• Simultaneous cult uprisings

• Attacks on orbital defenses

• Sabotage of pylon sites

• Refugees arriving from nearby systems

• Emergency mobilization across Cadia

• Conflicting orders from Imperial institutions

The transition from gathering storm to open war should feel like escalation, not a sudden unrelated event.

The Shape of the Siege

The siege of Cadia is larger than one assault upon one city.

It may include:

• Fleet battles

• Orbital bombardment

• Ground invasions

• Fortress sieges

• Pylon sabotage

• Cult uprisings

• Daemonic incursions

• Supply-line attacks

• Assassination missions

• Evacuations

• Counterattacks

• Fighting across neighboring worlds and stations

A party can influence the wider war by completing missions at critical points rather than commanding every army directly.

Cadia’s Defenders

The defense may include:

• Cadian regiments

• Planetary Defence Forces

• Imperial Navy fleets

• Adeptus Astartes

• Adepta Sororitas

• Inquisitorial forces

• Adeptus Mechanicus personnel

• Rogue Trader vessels

• Arbites units

• Militias and civilian volunteers

• Mercenaries and auxiliaries

These defenders may share one enemy while disagreeing over strategy.

Some demand that every position be held. Others prioritize evacuation, pylon defense, fleet survival, or preservation of key personnel.

Chaos Objectives

Chaos does not need to capture every fortress to win.

Its objectives may include:

• Breaking the Cadian Gate

• Destroying or corrupting the pylons

• Crippling the Imperial fleet

• Killing key commanders

• Isolating Cadia

• Opening stable routes from the Eye

• Recovering ancient technology

• Causing mass fear and psychic disruption

• Forcing Imperial institutions into conflict

Different warbands may pursue their own goals while contributing to Abaddon’s broader plan.

The Pylons

The pylons may become the most important strategic locations in the campaign.

Possible actions include:

• Defending active sites

• Repairing damaged structures

• Recovering control mechanisms

• Preventing sabotage

• Activating dormant networks

• Studying Necron connections

• Redirecting their effects

• Destroying a site before Chaos captures it

Using the pylons should involve risk.

Their activation may weaken daemons and psychic powers while disrupting Imperial psykers, astropathic communication, or Warp travel.

Civilian Survival

Cadia is not only a fortress.

It contains cities, families, workers, pilgrims, laborers, support personnel, and people who cannot simply abandon the world.

Civilian crises may involve:

• Overcrowded shelters

• Food shortages

• Broken transportation

• Cult infiltration among refugees

• Medical collapse

• Martial law

• Abandoned districts

• Evacuation lotteries

• Riots

• Officials concealing losses

Protecting civilians may conflict with military objectives.

Neither choice should automatically be presented as correct.

Evacuation

Evacuating Cadia would require more ships, time, and secure routes than the Imperium may possess.

Commanders must decide who or what receives priority:

• Civilians

• Wounded soldiers

• Experienced officers

• Regimental archives

• Pylon research

• Mechanicus personnel

• Sacred relics

• Industrial equipment

• Military reserves

A successful evacuation may save millions while leaving billions behind.

A failed evacuation can turn ports and orbital stations into traps.

Player Influence

The player characters can alter the siege by:

• Saving commanders

• Protecting pylon sites

• Destroying staging grounds

• Exposing infiltrators

• Securing reinforcements

• Resolving Imperial rivalries

• Recovering vital intelligence

• Defending evacuation routes

• Capturing enemy officers

• Negotiating temporary alliances

• Choosing which locations receive support

No single quest should decide the entire war.

Several successes or failures should change the available outcomes.

Measuring Campaign Progress

Franz can track the war through broad conditions rather than a fixed timeline:

• Imperial Readiness: troops, supplies, leadership, and fortifications

• Pylon Stability: how much of the ancient network remains functional

• Chaos Momentum: fleet access, cult activity, and captured objectives

• Civilian Survival: shelter, evacuation, and social order

• Faction Unity: cooperation among Imperial institutions

• Hidden Conspiracy: progress of the Fractured Sigil’s plan

These conditions can guide the ending without requiring visible numerical scores.

Possible Fate: Cadia Holds

Cadia may survive the invasion.

This could occur if the defenders preserve the pylons, maintain fleet support, prevent internal collapse, and inflict enough damage to force Chaos to withdraw.

Victory may still leave:

• Ruined cities

• Heavy casualties

• Political conflict

• Damaged pylons

• Isolated regions

• Hidden cult remnants

• A weakened but surviving Gate

Holding Cadia should feel costly rather than effortless.

Possible Fate: A Divided Cadia

The world may remain contested.

Imperial forces could retain fortress regions while Chaos controls other territories, orbital zones, or underground routes.

This creates a continuing war of:

• Sieges

• Raids

• Resistance cells

• Evacuations

• Shifting front lines

• Pylon conflicts

• Rival governments

Cadia remains alive but trapped in prolonged war.

Possible Fate: Evacuation and Strategic Withdrawal

Imperial commanders may conclude that the planet cannot be held.

The campaign may shift toward preserving troops, civilians, archives, relics, and pylon knowledge.

Cadia may be abandoned, destroyed, or left as a doomed rearguard position.

Survivors could establish new Cadian regiments, colonies, fleets, or governments elsewhere.

The loss of the world does not erase its people.

Possible Fate: Destruction of Cadia

Cadia may suffer catastrophic destruction.

This could occur through:

• Orbital attack

• Pylon failure

• Planetary fracture

• Warp instability

• Ancient weapons

• Deliberate sacrifice

• A failed attempt to stop Chaos

The characters may still influence who survives, what is recovered, and whether the disaster achieves Abaddon’s full objective.

The destruction of the planet should not automatically mean that every defender dies or every Chaos plan succeeds.

Possible Fate: Pylon Ascendancy

The pylons may be activated more successfully than in established history.

A powerful null field could weaken the Eye, trap fleets, suppress daemons, or temporarily seal routes.

Possible costs include:

• Psychic casualties

• Failed Warp travel

• Astropathic isolation

• Necron awakening

• Damage to reality

• Loss of Imperial control over the network

Saving Cadia through ancient xenos technology may create a new crisis.

Possible Fate: Transformation

Cadia may survive in an altered form.

It could become:

• A partially Warp-touched fortress

• A world protected by a permanent null zone

• An isolated system cut off from the Imperium

• A divided Imperial and Chaos battlefield

• A Necron-controlled strategic site

• A refuge governed by emergency military rule

• A symbolic capital for a new defensive alliance

Transformation allows the campaign to depart strongly from canon while preserving the importance of Cadia.

Possible Fate: Chaos Victory Without Occupation

Abaddon may achieve his strategic objective without ruling Cadia.

If the Gate opens, the pylons fail, or Imperial fleets are shattered, Chaos forces may move onward rather than occupying every city.

Cadia could remain technically Imperial while becoming strategically irrelevant or isolated.

This distinction prevents the ending from becoming a simple question of which flag flies over the planet.

Consequences Beyond Cadia

Whatever happens will affect the wider campaign.

Possible consequences include:

• New Warp routes

• Refugee fleets

• Imperial political blame

• Increased Chaos raids

• Broken trade routes

• Xenos intervention

• New military crusades

• Rise or fall of major factions

• Spread of the Hidden War

• Formation of new alliances

The outcome should create further adventures rather than end the setting.

D&D 5e Narrative Use

The siege can include:

• Battlefield missions

• Investigation

• Escort quests

• Sabotage

• Diplomacy

• Survival

• Dungeon exploration beneath fortresses

• Voidship encounters

• Mass-combat backdrops

• Boss confrontations

• Retreat and evacuation

The party should have meaningful objectives even when vast armies surround them.

The Siege in Play

This lore can support adventures involving:

• Defending a pylon complex

• Rescuing a trapped regiment

• Hunting a Chaos commander

• Securing an evacuation port

• Recovering command codes

• Preventing an orbital strike

• Negotiating with a xenos faction

• Exposing a compromised Imperial leader

• Carrying a warning through a communications blackout

• Choosing which fortress receives the last reinforcements

Cadia’s future is not predetermined.

The siege should respond to player decisions, faction actions, and the consequences of earlier missions.

The campaign is not about reenacting the fall of Cadia.

It is about discovering what Cadia becomes when the players are present.