Vital patches (such as version 2.6.x or newer) containing structural balance updates, performance patches, and foundational structural hooks required for Seasonal play.

The base file contains the core game data. For Diablo III, this includes the original campaign assets, textures, and audio files required to boot the game. 2. Update Files (NSP/NSZ)

The legitimate way to get Diablo III: Eternal Collection on Switch is:

The core file containing the main game data. The standard file format for Nintendo Switch backups is NSP (Nintendo Submission Package). On its own, the base game file will let you play, but you will miss out on crucial endgame features and class options. 2. The DLC Pack

Diablo III relies heavily on its DLCs for longevity. The Rise of the Necromancer DLC must be installed alongside the base NSP to unlock the blood-and-bone spellcaster class. Additionally, the Nintendo Switch specific Zelda-themed DLC is bound to these files. 3. The Latest Update (v1.0.x / Title Updates)

Which of those would you like?