The person who opened the door and came in was a complete stranger.
Literal
Door [object-を] open enter-came [nominalizer-の] [topic-は], completely unknown person was.
A cleft sentence: ~のは...だった focuses on identifying who came in. ~てくる on 入る marks the action as directed toward the speaker's location. 見知らぬ (unknown/unfamiliar) is a classical negative form of 見知る — literary but natural in narrative. まったく (completely) intensifies the strangeness.