Colossus

The Colossus is one of the few things in Stellaris capable of permanently reshaping the direction of a playthrough the moment it’s used — a devastating weapon capable of resolving a target planet in catastrophic fashion, but at the near-certain cost of provoking every other empire.

Unlock and build requirements

Unlocking the Colossus requires taking the Colossus Project ascension perk, then completing the special project it opens up — you’re prompted to choose your starting weapon when that project is halfway done, and can research and retrofit other weapon types later via the ship designer. Building one also requires a Citadel with Capital Assembly Yards (or a Mega-Shipyard), and you’re limited to one at a time (though it can be rebuilt if lost).

Confirmed weapon types include the Global Pacifier, which irreversibly converts the target into a Shielded world, and the World Cracker, which destroys the planet outright — different weapons carry different opinion penalties with other empires, and some have unique story consequences (using certain weapons on a Holy World, for example, can wake the Holy Guardians and trigger an immediate war).

Two ownership quirks matter before you commit. First, simply owning a Colossus hands you the Colossus casus belli — a total-war CB — against every empire, and hands every empire the Stop Colossus CB against you; if the ship is destroyed those CBs lapse, though wars already declared are unaffected. Total war means captured systems annex immediately with no claims involved, which cuts both ways. Second, the hull has zero ship-to-ship weapons: in a battle it ignores the fighting and flies straight at its designated planet unless set to Evasive stance, so it always needs a real fleet escorting it.

You’re also not the only one who can field one. Each Fallen Empire has a 10% chance to start the game with a Colossus, and awakened empires can build more. If you integrate a subject that owns one, the ship transfers to you and bypasses the one-per-empire limit — though bonus hulls acquired that way can’t be rebuilt if lost.

The weapon lineup

The starting weapon you pick at the project’s midpoint is only the beginning — the full roster covers very different flavors of planetary removal, each with its own availability rules and opinion costs (penalties from repeated use stack, up to five times):

• World Cracker: shatters the target (ruining ringworld segments and outright removing habitats); former habitable planets gain 4-16 minerals in deposits. Blocked for Pacifists. Around −1000 opinion with the victim and −100 with its federation — the harshest in the roster. Fanatic Purifiers and Determined Exterminators gain 600-3000 unity on first use, and it can be test-fired on uninhabited planets. Shattering Earth has a 50% chance to spawn a wormhole pair.
• Global Pacifier: seals the planet under a permanent shield, adding a 10-society-research deposit, without killing anyone. No ethic restrictions, and the mildest reaction: −200 with the victim, −20 with its federation.
• Neutron Sweep: kills every pop but leaves the planet colonizable at −30% habitability for 10 years. Blocked for Pacifists (Spiritualists on the psionic path get the Divine Enforcer instead). It also cleans Infested Worlds into barren worlds with a terraforming candidate modifier — a genuine anti-crisis tool. −800 with the victim, −80 with their federation.
• Divine Enforcer: converts all pops to Spiritualist while killing mechanical, machine, and hive-minded pops. Requires Spiritualist ethics plus a psionic ascension route. −400 with the victim, −40 with their federation.
• Nanobot Diffuser: Driven Assimilators only. Assimilates every biological pop (they gain the Cybernetic trait), kills machine and hive pops, and transfers ownership of the planet to you. −600 with the victim, −60 with their federation.
• Later additions round out the roster: the Deluge Machine (tied to the Hydrocentric ascension perk — kills non-aquatic pops and terraforms the planet into an Ocean World), the Devolving Beam (Ancient Relics — devolves biological pops into pre-sapients), and the Planet Smelter (thermophile-flavored builds — cooks the target into a Volcanic or Molten world).

One warning overrides all of the above: firing a planet-killer on a Holy World instantly awakens the Holy Guardians and sends them at you in a war — the single fastest way to turn a comfortable mid-game into a survival scenario.

What to consider before using it

• Diplomatic cost: using a Colossus will almost certainly tank your standing with every other empire, potentially triggering a large-scale coalition against you.
• Irreversibility: the effects are typically permanent. Make sure this is genuinely the strategic tool you need for this playthrough before pulling the trigger, not a snap decision.
• The victim radicalizes: a planet-killer strike piles war exhaustion onto the target empire and pushes its ethics attraction toward Militarist and Xenophobe, hardening the whole neighborhood against you over time.
• Pre-FTL targets are still an incident: firing on a primitive world requires owning the system and an aggressive interference policy, and it still costs roughly −40 opinion with most empires (−60 with Xenophiles).

When it’s actually worth considering

For dealing with an extreme threat (specific story branches, or if you’ve already committed to a “galactic scourge” playthrough), the Colossus can be an extremely high-value option. For a standard playthrough where you care about normal diplomatic relations, exercising restraint is the safer call.

It’s also the cleanest answer to a few specific problems: a Neutron Sweep sterilizes an Infested World into something terraformable, and total-war terms let you erase a rival’s core sector in one war instead of three claim-limited ones. If the galaxy already hates you — genocidal builds, crisis ascension runs — the opinion penalty is a cost you’re paying anyway, which makes the Colossus close to free value.

FAQ

Which DLC do I need for the Colossus? The Apocalypse expansion is the classic source, and current versions also open it with The Machine Age — with either enabled, the Colossus Project ascension perk is available to pick.

Can I rebuild a destroyed Colossus? Yes. The limit is one at a time, not one ever. Captured hulls from integrated subjects and unique Colossi (like the Toxic Entity or Cetana’s Colossus) don’t count against your limit, but they can’t be rebuilt if lost.

Does the Colossus fight fleets? No — it has no conventional weapons at all. Never send it anywhere without an escort, and consider Evasive stance so it stops diving through battles toward its target.

Can I change its weapon later? Yes. Research the other planet-killer technologies as they appear and retrofit the hull in the ship designer like any other ship.