Umar first established himself at Zabid, then moved into the mountainous interior, taking the important highland centre Sana'a. However, the Rasulid capitals were Zabid and Taiz. He was assassinated by his nephew in 1249. Omar's son Yousef defeated the faction led by his father's assassins and crushed several counter-attacks by the Zaydi imams who still held on in the northern highland. Mainly because of the victories he scored over his rivals, he assumed the honorific title "al-Muzaffar" (the victorious).