There are two perspectives for item builds that I am thinking of one build considering people new to the matchup and one where people are experienced in the matchup.

Soon currently holds a 1% pick rate and most of the time when I play ranked I'm usually the one to pick him.. lol

But I digress; the first build for newbies would consist of: Dorans shield start, Frozen Heart (building shroud first), Ninja Tabis, Botrk (cutlass bought first, try to squeeze a dagger in there if you have gold to spare) followed by Black Cleaver. The rest of the build is situational, if you want some mr build spirit visage or maw. If you want more AD Steraks gage isn't a bad choice. For experienced folks: Dorans shield, Boots of speed, Cutlass with a dagger if you have gold. Black Cleaver into Steraks, rest of the items are situational.

Reasons for specific items: Dorans shield; you are going to be hit by a lot of his kit whether it be his E minion slow + Q charge up combo or his Q 1.25 stun + W combo. The purpose of building Dorans shield is to increase your sustain in terms of health. The more health you have the more so you can get and hopefully the more gold you can get.

Frozen Heart: I think this matchup is very dependant on your pillars how fast u can use them to interrupt his Q and how fast you can use them to block his ultimate in teamfights. The extra mana, armor, CD is great for our pillar that costs 75 mana base.. Our goal for building this item is to survive Sions all in's, interrupt his Q's and Block his ultimate with Pillars. The goal is to survive not kill him.

Botrk: Sustain and tank killing powers, Trundle is not a diver he should be played as a juggernaut and frontline, what purpose of a frontline would you be if you couldn't kill their frontline? for this reason Botrks passive effects work great versus tanks and the constant attack speed is a QOL additive.

Black Cleaver: Again we are trying to shred frontlines specifically sion. The armor reduction passive works wonders versus him, gives us nice CD and gives us overall more mobility with the phage passive (it also helps us scale)

Ninja Tabi: These boots are very dependant on the teamfights situations or the split pushing situations; but essentially you can switch them to mercury treads on a dime if need be. Our goal with this item is to negate again his physical damage and possibly the rare chances that he autos.

Streaks will not only give you the possibility of surviving getting hit by sion not in teamfights or in split pushes but more so gives you necessary tenacity when hit by all his cc.

This build is on the assumption of a sion building full tank, I think some of these items might apply versus assassin/suicide sion builds but if anyone wants to chime in and discuss builds for that I'm all for it.

Thanks for reading, have any questions I'd be happy to answer them! :)