Ἀλλ' Ἀχέροντι νυμφεύσω → I will become the bride of Acheron
dat. de ὑμεῖς.
irregular dative case of ὑμεῖς; to (with or by) you: ye, you, your(-selves).
ὑμῖν: dat. к ὑμεῖς.