From Halopedia, the Halo wiki
The Yappening is the name a humourous in-game event occaisionally created by 343 Industries around April 1. The event has occurred in 2018 and 2019, with the 2018 event taking place in-game on Halo Wars 2 and the 2019 event in Halo: The Master Chief Collection. The Yappening is fictionally (albeit non-canonically) framed through the lens of Yapyap the DESTROYER gaining access to 343's servers and using it to wreac havoc in the games. The events typically come with limited-time changes to game functionality, often for comedic effect.
Halo Wars 2 (2018)
On March 28, 2018, a blog post was submitted to Halo Waypoint by Yapyap, detailing that he had found a Forerunner artifact called the "Key of Balance", which promptly began speaking to him about the responsibility of balance. Yapyap proceeded to talk with the key and eventually destroyed it, using the key to bring about change. This post was updated by Jeff Easterling purporting that the post was not authorised by anyone on the Waypoint team, then later again updated noting that the event had also affected Halo Wars 2 game files, with a list of patch notes detailing the limited-time event changes over the weekend. The effects given initially only affected unit balance, but went on to affect leader powers, skirmish, Terminus Firefight, matchmaking, playlist names and the campaign level Under the Dark. Notably, the changes in particular affected Yapyap's gameplay, giving massive buffs to his units and starting army, culminating in Yapyap being given his own Season.
Halo: The Master Chief Collection (2019)
The Yappening returned again on April 3, 2019, this time nicknamed The Yappening: Two Yaps This Time. A blog post was once again posted on Waypoint by Yapyap, detailing how the Grunt had remembered the event a year ago, and decided to repeat it. His troops brought him several items (themselves tongue-in-cheek references to existing lore) such as Janus Keys, slipspace flakes, a "Fragment of Divinity" and pistachios, with Yapyap failing to get them to yield any tangible result. Yapyap then destroyed the assembled artifacts with plasma grenades, resulting in a mysterious voice conferring Yapyap as the true protector of the Mantle, among other grand titles. The event then sparked a similar series of events internally at 343 Industries, resulting in a dedicated Yappening splash screen, menu art, nameplates and playlist being created inside The Master Chief Collection 2v2 Halo 2 matchmaking. The Yappening playlist consisted of several unique gametypes;
The Yappening nameplates were unlocked as follows;
The Yappening update accompanied a series of regular title update changes for Master Chief Collection, detailed in the same blog post.