That's right -- it's inflected for the relative pronoun was.
Compare: Was wird passieren? to Ich weiß nicht, was passieren wird.
Or in English: "What is going to happen?" to "I do not know what is going to happen." -- no infinitive for "is going to"; it's third person singular, matching "what".
A relative clause is one that employs a relative pronoun or adverb, that is a pronoun or adverb that refers to an element in the previous sentence (antecedent) but is used instead of repeating that word. It may be clearer with examples:
in “everyone who knows you loves you”, “who knows you” is a relative clause where “who” is a relative pronoun referring to “everyone”;
in “I have a cat that never purs”, “that never purs” is a relative clause where “that” is a relative pronoun referring to “cat”;
finally in “I remember the place where we met”, “where we met” is a relative clause where “where” is a relative adverb referring to “the place”.
Relative clauses are by definition subordinate clauses (“Nebensatz”, pl.: “Nebensätze”) and as such follow verb-final word order.
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