Sion decimating smash...

So i havn't played new sion yet so i wan't to ask this instead of jumping to conclusions.

So i have watched people play and read his abilities and such but this one time in particular in my last game i was quite confused. Sion had come up to me and my friend who i was in vent with and started charging his decimating smash the full way. we both just stepped to the side to avoid it but then sion's character model just moved position and the entire decimating smash moved over without any warning and did full damage. we both got hit for 500ish damage which ultimately cost us our lives.

but we were both quite stumped about this, so... Is that intended or is that a bug?