I may be wrong but karthus was nerfed years ago because of a similar problem sion has right now.

Years ago when the world of runeterra was new, and seen as a cheep knock off of Dota, and before everyone was challenger, karthus still had his same passive.... with one exception. Back then he could auto attack in his death form.

"But karthus is so useless in his auto attacks it was probably a buff so it forced players to use his abilities when in his passive" Is what you might be thinking. If this is what you were thinking, then yes you are right. It was an overall buff to his champ because it did force people to use higher damage. However it was a nerf to a cheese style tactic that was considered too good back then.

The fact Karthus could auto attack in his passive meant he got 5 seconds of being a near perfect ADC because he couldnt be targetted and stopped from auto attacking, this was very far from popular and definitely a cheese tactic, but what made it worse was the INTING Karthus strat. This was back when we had the ultra revive/tp cheese, a karthus could just sell all his items for AD and push down towers with little worry. You could use it when you were ahead for tons of pressure, or when you were behind as a last ditch effort to keep the enemy away form your base/ getting that one last push that might win you the game.

AGAIN I WILL SAY THIS, IT WAS FAR FROM POPULAR BUT STILL EXISTED AND WAS SEEN. I did win a game this way all those years ago...

Now while we cant remove auto attacks from sions passive, however, I do think turning its focus to be more champion centered would help resolve this issue of Inting sion. Maybe give it conditions like deals 75% less damage to enemy objectives (epic or jungle creatures or minions) and increased damage from turrets if not targeting an enemy champion. Maybe add an extremely small damage return on hit from his shield to draw tower aggro? At this point im sort of spitballing ideas to help draw away this rather poor gameplay idea from standard use.

Lets be honest, the sion either has so much presence one team find themselves so far behind. Either hes as useless as I am on ADC or he completely runs over the enemy team like in C9 vs AF (or FNC vs C9 TT_TT)

Ive had games go both ways so im not that big of it both being on my team or being on the enemy team.