Yorick's ult breaks Sion's passive.
I am an avid Yorick player. I noticed the other night when I ulted Sion to take down a tower and he died several seconds later, he simply flat out died. He did not proc my ghost's effect, nor his own passive. This appears to occur whenever Sion dies after I ult him, whether he is still under the effect of Yorick's ultimate or not.
ult on him yet? I'm wondering if it has a similar result...