Sion's Decimating Smash does seem to be bugged.

So a friend of mine mentioned this to me, and I saw some bug reports mentioning the same issue getting downvoted. So I hopped in Practice mode and tested some stuff, and he does indeed seem to be bugged.

After using Decimating Smash, Sion is unable to move or cast anything with a cast time for ~ 0.5 seconds.

I've tried to use R, E, Ravenous Hydra Active, Protobelt and ''Walk'' after using Q, Sion seems to be locked out of all of these for 0.5 seconds. I also tried it with Youmuus Ghostblade because it does not have a cast time, and this Seems to be working fine.

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The Friend who told me about this just mentioned it happens on his E, Roar of the Slayer, aswell, but I have not tested this.