A rift tears open on the road just outside Riverwood, and three of World of Warcraft's Ghouls come out of it to fight you.
What you get
The rift: a blazing portal on the Riverwood road, right where a new game's travels begin.
WoW's Ghouls: World of Warcraft's real Ghoul model and skin, moving and fighting with Skyrim's draugr animations.
WoW loot: each Ghoul wields a real World of Warcraft weapon (Ghoul Hammer, Ghoulslicer and Corpse Harvester) with its own name, model and texture. Loot it from the body and fight with it yourself.
Crimson Desert's battle theme joins Skyrim's combat music while you fight near the rift.
Single player.
What you need
Skyrim Special Edition, World of Warcraft (retail) and Crimson Desert, all installed. Press Play: the first time, the mashup builds the Ghouls, their weapons and the music from your own copies of the games (it takes a minute), then Skyrim starts. Nothing from any of the games ships with it.
Credits
The Elder Scrolls V: Skyrim Special Edition (Bethesda Game Studios), World of Warcraft (Blizzard Entertainment), Crimson Desert (Pearl Abyss): played from your own copies.
CascLib by Ladislav Zezula (MIT): reads World of Warcraft's files.
vgmstream by the vgmstream team: decodes Crimson Desert's audio.
LZ4 by Yann Collet (BSD-2) and zlib by Jean-loup Gailly and Mark Adler.
Ultimate ASI Loader by ThirteenAG, installed by Melty.
Crimson Desert archive layout as documented by NattKh's CrimsonGameMods; Skyrim model format from the NifTools documentation.