More fantasy/game narration. 十字キー ('cross key') is the Japanese term for a gaming controller's D-pad (directional pad) — literally 'cross-shaped key.' BGM is the Japanese-English abbreviation for 'background music,' read as 'bi-ji-emu.' すると ('and then, whereupon') is a narrative connective showing sequence. The final noun-phrase with trailing へ and ellipsis creates suspense — the change is happening as the speaker describes it. Typical of an excited retro-game commentary.