How did Yazeed bin Muʻaawiyah die? How did ʻUbaydullaah ibn Ziyaad die?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

As for Yazeed ibn Muʻaawiyah, Ibn Al-Atheer mentioned the cause of his death in his book Al-Kaamil among the deaths of the key Muslim figures that occured in 64 AH. He wrote, "The death of Yazeed ibn Muʻaawiyah was reported to have taken place in that year. He was injured in the side of his face by a stone thrown by a catapult. He fell ill for some days and then passed away." [Al-Kaamil]

As for ʻUbaydullaah ibn Ziyaad, Ibn Al-Jawzi mentioned the cause of his death in his book Al-Muntathim:

"Among the events that took place in 67 AH was the death of ʻUbaydullaah ibn Ziyaad. Ibraheem ibn Al-Ashtar led his army to fight against ʻUbaydullaah ibn Ziyaad and the two armies met near al-Mawsil (Mosul). The battle was fierce and both parties lost a great number of men. Ibn Al-Ashtar said, 'I killed a man and I smelled the scent of musk from him under a banner by the river; so go find who he was.' It turned out to be ʻUbaydullaah ibn Ziyaad. Ibn Al-Ashtar hit him with his sword, splitting him into two halves." [Al-Muntathim]

Allaah knows best.