There is no official Undertale DS version released by Toby Fox. Any “Undertale DS ROM” circulating online would be a fan-made port or a hacked reproduction, not a legitimate Nintendo DS game.
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Download the patcher and project files from the C3RV1 UndertaleNDS GitHub . Locate your official Undertale installation folder on PC. There is no official Undertale DS version released
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This is the most prominent project specifically for the original Nintendo DS and DSi. It is a "demake" that aims to recreate the game from the ground up for DS hardware.