A file with .dek extension is a deck of cards file used by the online collectible car game, Magic: The Gathering Online (MTGO). Though in more cases, these deck of cards are playable, in some cases, DEK files list a non-playable collection of cards. These can’t be played but act as a subset of a collection. DEK files can be created from CSV files by exporting using Delver Lens and can be used in Tabletop Simulator.
DEK File Format
DEK files are stored as multiline plain text files in XML format that are human-readable. These can be opened with any text editor such as Microsoft Notepad, Notepad++, and Wizards of the Coast Magic: The Gathering Online. DEK files are also used by many Magic players to import/export from many deck-building and websites such as CardHoarder. These websites allow to upload MTGO DEK files as a single or list of files.