[@Kho]

Depending on view, Husayan ibn Ali was killed and himself beheaded after being killed by Hurr ibn Yazid Riyahi at the [url=https://en.wikipedia.org/wiki/Husayn_ibn_Ali#Battle_of_Karbala]Battle of Karbala[/url]. Shia view attests he and his followers were executed and decapitated, where the Sunni view simply makes the claim Husayan was killed by Hurr's spear before being beheaded.

And in a half-sense, Rostam Farrokhzaad was beheaded after the load of him camel was cut, crushing him under his weapons at the final stage of the [url=https://en.wikipedia.org/wiki/Battle_of_al-Q%C4%81disiyyah#Final_battle]battle of al-Qaadisiyysah[/url].

So it's not like it  hasn't happened nor has had its precedent during the Rashidun era.

And yes, the Rashidun are already moving into the Ghassanid lands, or already well in it.