As much as Lady Rastan's concentration seems to be fixed on whatever it is that she's looking down on, simply starting forward doesn't seem like the smartest approach.
You crouch down, gazing intently at Lady Rastan. After only thirty seconds, you can see that she is going through waves of concentration, at times thoroughly absorbed, while more aware of her surroundings at others.
Picking your moment carefully, you start forward, trusting in the fact that Lady Rastan is having to devote her full focus to her incantation as you do so. You seem to have chosen well: Lady Rastan gives no sign of having heard your approach, and you can now hear words tumbling out of her mouth, thick and fast.
When you're barely more than a foot from her, her concentration lapses, and before you can react yourself, Lady Rastan has whirled around, an expression of pure shock on her face. Then she stumbles back, eyes wide—
—and finally falls backwards, tumbling over the edge.
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