The Fulmina Disc is an old disc containing genetic data used to resurrect the CHAOS siblings (Agita, Anima, Fulmina, Gurgita). It can be found in one of the computers at Precursor Valley Dark Cave - Laboratory.
There are three other CHAOS discs besides this one: the Gurgita Disc, the Anima Disc and the Agita Disc. The player can take them to the secondary laboratory at Parum City. If we talk to the scientist from the The Extinction Project quest after collecting the four discs, he'll resurrect the CHAOS siblings and they will start roaming at the Precursor Valley.