Temporary modulation to bIII (flattening the key by 3 flats) via a ii - V - I approach.
In terms of the main key, the progression is ivm - bIIx - bIIIM.
The first two chords of this modulation look like the start of a plagal cadence, an ambiguity that is used in Witchcraft, for example, which uses them both ways.
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