Zombie Invasion is a recurring event where invading zombies spawn across the World Map. Killing them earns Courage Medals, redeemable at the event's trade store, and occasionally spawns a powerful Zombie Boss that requires a rallied group to take down.
Once your server passes Season 2, defeating these Zombie Bosses also feeds into a separate alliance-wide challenge called Glacieradon - see the dedicated Glacieradon guide for that part. Everything below covers the base event itself.
How it works
- Killing invading zombies earns Courage Medals, spendable at the event trade store.
- There's a random chance a Zombie Boss spawns after a kill - these are much stronger and need a rally.
- Damage dealt to Zombie Bosses is tracked on a leaderboard; higher cumulative damage earns better end-of-event rewards.
- Everyone who helps defeat invading zombies gets an Invasion Defender reward once the event ends.
- A freshly-spawned Zombie Boss has a 10-minute protection window where only the discovering alliance can rally it.
How Zombie Boss levels actually work
Every Zombie Boss you spawn is tied to your own personal "summoning level" - not a server-wide difficulty setting, and not tied to your Power or your base level. Two players on the same account age can be spawning very different boss levels depending on their own history.
The formula
Whenever you spawn a Zombie Boss, the game picks a random level somewhere in a window around your summoning level: from 15 levels below it, up to 5 levels above it. Higher levels within that window are rarer than lower ones.
Everyone starts at summoning level 5, and it can only ever go up - there's no way to lower it once it's risen.
What actually raises your summoning level
Two things move the number, and only two:
- A boss above your current summoning level gets killed - it doesn't matter who lands the kill, or whether you were even in that rally. If it happened, your summoning level jumps straight to match that boss's level.
- You personally lead the kill on a boss at or above your summoning level - this adds a flat +5 to your summoning level, and can only trigger up to 3 times per event window.
A worked example
Say your summoning level is 25. Until something changes it, your spawns will land somewhere in the 10-30 range, weighted toward the lower end.
- If any summoned boss of yours at level 30 gets killed (by you or an ally), your summoning level jumps straight to 30 - your future spawns now range from 15-35.
- Alternatively, if you personally lead a kill on a boss at level 25 or higher, your summoning level rises by 5 to 30 - same new range, different path to get there.
Why this matters for your alliance
Because rule #1 doesn't care who does the killing, your alliance's strongest players hunting high-level bosses quietly drags EVERYONE's summoning level up over time - including members who never touched a high-level boss themselves. If your alliance has a mix of heavy spenders and newer/weaker members, it's worth talking about this openly: a few members chasing big bosses can make the event genuinely harder for everyone else within days, with no way to undo it.
A note on reliability
Worth knowing honestly: even though this is the official explanation from the game's own support team, plenty of players report their actual spawns don't always line up with it - some see boss levels well outside the expected 15-below-to-5-above window. Treat the formula above as the best available explanation, not a guarantee - if your results look inconsistent, you're not alone, and it's likely not something on your end.
Trade store
The event store lets you redeem Courage Medals for a rotating set of items - some only unlock once your server reaches a certain age (for example, Oil becomes available once your server enters Season 3 pre-season).
A solid default spending order for free-to-play players:
- Stamina
- Legendary Gold Chest
- Universal Exclusive Weapon Shards
- Drone Parts
- Purple Gold Chest
Practical tips
- There's no fixed number of zombies you need to kill to spawn a boss - it's random, so don't wait around expecting one on a schedule.
- The in-game "suggested power" indicator isn't always reliable for predicting whether you can beat a boss - when in doubt, gather your strongest available allies and try.
- Coordinate with your alliance before attacking someone else's discovered Zombie Boss - many servers have their own etiquette rules about this beyond the built-in protection window.