>>55142774Their elements. They aren't immortal per say, more like their true form is a concept so farfetched it cannot be erased. Even if Dialga died, his true form is the embodiment of time, and a new Dialga would be born with time passing. Same for Palkia and Giratina albeit Giratina dying while a portal is open would mean the distortion world expanding infinitely and eventually collapse reality, which also means death for Giratina.
Uxie, Mesprit and Azelf die with humans, they are concepts that embody humanity. The rest of the legendaries can all be killed, albeit too hard to do so. Ho-oh would probably rise from his own ashes. Killing Reshiram and Zekrom would probably revert them into their sphere forms, a lot of mons have recovering forms like that, Cosmog, Terapagos, Jirachi, etc
Kyurem is already dead, it is an empty vessel of the God Dragon left behind from the separation of Resh and Zek, probably safe to say he is immortal.
Xerneas and Yveltal also have resting forms
