It seems to me that more and more games in the last few years feature an invasion mode for their campaigns, in which another player (usually in a hostile role) enters the single player campaign mode of someone else.
When was this game mechanic of online (or maybe offline) invasion of an otherwise single player mode first introduced in video games?
The feature you're referring to is called invasion multiplayer.
The game that introduced this seems to be Demon's Souls from 2009 (lauded for introducing the core characteristics of the Souls game series):
Online multiplayer allows both player cooperation and world invasions featuring player versus player combat.
source
It is worth nothing that this gameplay mechanic was being developed two years earlier by Arkane, but the game which had it implemented was never released:
In 2007 Arkane Studios began creating a game called The Crossing [..]. The team behind the game wanted to include a mechanic they called “cross-player”, in which host players would have some enemy characters in their game controlled by other human players and give players a reprieve from the usual multiplayer experience.
Unfortunately, the game was never released due to publisher and financial issues, but the cross-player mechanic was definitely a predecessor to invasion mechanics in future titles.
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Arkane did pick up some core elements of The Crossing for Deathloop.
Games that offer invasion multiplayer are:
| Title | Year | Type |
|---|---|---|
| Elden Ring | 2022 | ⨁⨺ |
| Sniper Elite 5: France | 2022 | ⨁⨺ |
| Deathloop | 2021 | ⨺ |
| Demon's Souls | 2020 | ⨁⨺ |
| Dark Souls: Remastered | 2018 | ⨁⨺ |
| Watch_Dogs 2 | 2016 | ⨁⨺ |
| Dark Souls III | 2016 | ⨁⨺ |
| Bloodborne | 2015 | ⨁⨺ |
| Metal Gear Solid V: The Phantom Pain | 2015 | ⨺ |
| Watch_Dogs | 2014 | ⨁⨺ |
| Dark Souls II | 2014 | ⨁⨺ |
| Dark Souls: Prepare to Die Edition | 2012 | ⨁⨺ |
| Journey | 2012 | ⨁ |
| Dark Souls | 2011 | ⨁⨺ |
| Demon's Souls | 2009 | ⨁⨺ |
⨁: helpful ⨺: hostile
Since you included the possibility of "invading" an offline game... I want for sake of completion to point out that the idea of "forcing" your way into someone else game and potentially steal their progress was seen in at least another very famous series far before Demon Soul was even in the work.
The older gamers may remember the "Here comes a new challenger" feature in Street Fighter II that allowed any challenger to join the game at any time, interrupting whatever the first player was doing and pulling them in an one-vs-one match: the winner would get to continue playing while a lost would mean a gameover. This fits the idea of "invading" the game since the competitive mode could be accessed at any time without the consent of the other player.
I don't know if other games before Street Fighter used that idea too, but the words "Here comes a new challenger" have become so popular by now that basically everyone takes this game as the first example of an arcade game that could be "invaded" by a challenger at any time.
