Complex sentence: During the hiatus, Malakian forming a band called Scars on Broadway, which was joined by Dolmayan, which after one "self-titled album" became dormant and Dolmayan has since left the band.
Split into simpler sentences: During the hiatus, Malakian formed a band called Scars on Broadway, which was joined by Dolmayan. After one "self-titled album" the project became dormant and Dolmayan has since left the band.