Complex sentence: Thestrals are winged horses introduced in "Harry Potter and the Order of the Phoenix" who have seen a death, and even then only after the death sinks in to become an established fact in their minds.
Split into simpler sentences: Thestrals are winged horses introduced in Harry Potter and the Order of the Phoenix. They can only be seen by people who have seen a death, and even then only after the death sinks in to become an established fact in their minds.