A slant-rhyme or half-rhyme by definition is an approximate rhyme not a true rhyme, so it depends how close you want these two words to sound. In my opinion, they are not close due to different syllable counts. Exterior has four syllables (ex-te-ri-or), while terror has only two (ter-ror).
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sailsofoblivionAre terror and exterior slant rhymes?Yes. Google "slant rhyme examples".