The datamine for the upcoming Crown Tundra reveals a new item. While its name isn't known yet, it's effect is coded. It allows a Pokemon with one of its normal abilities to permanently switch over to its Hidden Ability. For those who play in non-official formats, that makes any Gen III and IV exclusive moves now compatible with Hidden Abilities, like Soft-Boiled Clefable now being able to have Unaware(It cannot learnt the Soft-boiled TM in Gen I, for the record, only the Gen III Tutor), and the Pokemon XD Psycho Boost Lugia can now have Multiscale. It also allows many event Pokemon, past and present stuck with their much worse normal ability to have their Hidden one, like the massive amount of Static Pikachu events.
If hacked into your inventory it is fully functional, but without a proper menu sprite or name.