Invasion

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For the 1997 megawad, see The Invasion.
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Invasion is the name of a popular game mode for multiplayer Doom source ports, present in Zandronum and ZDaemon. It is a variant of the cooperative (or survival) game mode commonly played in multiplayer Doom, however it presents some key differences. It is played on specialized maps, where players will be presented with initially empty maps that get populated with monsters over time. These monsters spawn in waves, and killing all monsters from a specific wave allows players to advance to the next one. Items and weapons are also spawned alongside the monsters, and in many cases, portions of the levels also get slowly opened up as each monster wave is vanquished. This results in a unique progression system compared to the exploration-focused nature of most Doom levels, and also encourages a strongly defensive playstyle, as players risk being overwhelmed by the hundreds of monsters that spawn if the waves are not thinned rapidly. Skulltag was the first port to implement the invasion game mode.

Odamex does not support Invasion mode directly, however it implemented the Horde mode, which is similar.