A lot of time has passed in-universe between the events of the original Hades game and Hades 2. For one, Melinoe was born from Persephone and Hades and has now basically fully grown up. Two, Chronos has overtaken the House of Hades and (assumably) taken Melinoe's family hostage. And three, the rest of the Greek Pantheon has been fighting against Chronos or working directly with Hecate and the rest of the rebels at the Crossroads to aid Melinoe in her impossibly difficult task.