When was the game mechanic of online invasion of single player mode first introduced?

[Alberto]

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?

[Laycee]

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.
^{https://en.wikipedia.org/wiki/Demon%27s_Souls}

It is worth nothing that this gameplay mechanic was being developed two years earlier by Arkane, but https://en.wikipedia.org/wiki/The_Crossing_(video_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.
^{https://gamerant.com/invasion-mechanic-needed-more-games-dark-souls-deathloop-good/}

Arkane did pick up some core elements of The Crossing for https://www.ign.com/articles/deathloops-invasion-pvp-mode-can-be-played-like-a-co-op-game .

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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

^{Sources: https://www.mobygames.com/game-group/gameplay-feature-invasion-multiplayer , https://www.ign.com/articles/elden-ring-multiplayer-co-op-pvp , https://gamerant.com/deathloop-julianna-invasions-better-dark-souls/ , https://en.wikipedia.org/wiki/Watch_Dogs_2#Gameplay , https://en.wikipedia.org/wiki/Journey_(2012_video_game)#Gameplay}