1.0 world eventFimbulvinter
Jotun Invasion · up to 3 at once · no timerAn invasion site is the only place 
Malicious Blood exists, and 3 of those are what summon Kall Fimbulbringer, so the last boss of the game is gated behind this event rather than behind a biome. Breaking the Malicious Ice at the end of a Morkhalla starts one.
It does not land in the Deep North. An invasion can only appear in Meadows, Swamp, Mountain, Black Forest or Plains, which is the half of the world you already live in, somewhere between 1 km and 12 km from the centre of the map, and it marks itself on the map as Jotun Invasion. Nothing times it out. It sits there, snowing, until somebody shuts it off.
Both ends of the event are the same object wearing two names. A block of Malicious Ice with 1000 health sits in the end room of a Morkhalla; break it and the server sees "The Jotun Advance" and an invasion appears elsewhere in the world. A second block, the same 1000 health, sits at the centre of the invasion itself; break that one and everyone sees "The Jotun Retreat" and the site is removed.
Three can run at the same time and no more. Breaking a fourth block of ice in a Morkhalla while three invasions are live does nothing at all, so clear one before you go looking for the next. Placement is fussy in two more ways: it refuses any spot within 450 metres of a player, and it refuses ground under world height 35, which is 5 metres above the waterline.
The waystone is the part that matters. It is one core with seven blocks of black ice set around it, and those blocks are where the 
Ice comes from; the rest of the structure is plain ice dressing. The shelves only generate on water, up to 800 of them, so an invasion that lands on a coast or a lake looks nothing like one inland. The event also forces its own weather across the whole radius and swaps that weather per biome, so a Plains invasion and a Swamp invasion do not look the same.
The core is a spawner as well as a target. It wakes at 120 metres, then tries one spawn every 50 seconds inside 10 metres of itself, holding 6 nearby and 50 in total. Each one has a 15% chance to come out starred, up to two stars. Half the rolls are a plain Krigen and a quarter are the dual-wielding version, which is why three quarters of what walks at you is a Krigen. The spawner stops when the core does, so the fight has a finish line.
One core is one Malicious Blood, every time, and the altar wants 3. That is 3 full invasions: an end room to break into, a site to find and a core to bring down, each time. Kall Fimbulbringer is what the offering buys.
The chain runs through the Deep North: Morkhalla for the ice that starts it, the Winding Tunnels for the moulds, and the Deep North page for everything else the biome holds. What 1.0 added covers the update as a whole.