。
A dark shadow is falling over me.
Literal
I [to-は] dark shadow [subject-が] cast-is.
さす (差す) has many meanings; here it's the 'cast/shine' sense for light, shadows, colors — 影がさす = 'a shadow is cast.' The ~ている marks the ongoing state. A metaphorical expression for impending misfortune or gloom, though can also be literal. Has a poetic/literary ring.