Boss waves and the Behemoth
Waves in FortWar come every 40 seconds, and most of them are about numbers. Every 15th one is about one monster.
The Behemoth
Every 15th wave (about every ten minutes) and every horde surge brings a Behemoth. It's huge, slow and tough, and its health scales with your fort's threat like every other monster's, so an old fort's boss is a much bigger fight than a new fort's.
It arrives with a red ๐ BOSS WAVE banner, and while it's alive a big health bar with its name runs across the top of the screen (and across the stream overlay).

The boss arrives with its name bar, and the red ring is the slam coming.
Step out of the ring
Every 6 seconds the Behemoth slams the ground. First a red ring flashes around it for 1.2 seconds. Then everything inside the ring takes a heavy hit: buildings, your commander, your troopers, viewers' avatars.
So when you see the ring, step out. That's the whole skill, and it feels great when you get it right with the boss right on top of you.
The roar
At half health it roars, and 8 crawlers join it. If you were fighting it at the walls with everything you had, that's the moment the walls get busy.
Kill it and you get a green ๐ BOSS DOWN banner, a toast and a line in your replay. It's worth 60 crystal.

Chat names it
This is my favourite bit. Before the boss arrives, viewers can type !boss <name> to suggest a name. When it spawns, one suggestion is picked at random and shown upper-cased with the credit: "๐ GRONK (named by sam)". Over the boss, on the boss bar and in the banners.
So the thing trying to flatten your fort was named by someone in your chat. Every boss fight ends up with a story attached.
Names go through the same name filter as viewer names, so nobody gets to put something nasty over the boss. With no suggestions, it gets a built-in name. Naming works whether chaos is on or off: it's just a name, and the boss comes either way.
Bring a satellite
Chat's orbital strike does triple damage to the Behemoth. If chat has the ๐ฐ meter full when the boss shows up, that's the best use of it in the game.
Go fight one
Your first boss comes at wave 15 at fortwar.io. If you stream, tell chat to start naming it early. And if you think of a better boss move than the slam, bring it to the stream or the Discord.
โ Jeff