Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
I like playing Sion as a half tank and half DPS. this works well for some people, but not all. I think his skills enrage and cannibalism really helps him become s DPS without much items so his items can also focus on armor. I suggest you try my build out because when it works it is very fun to play. :)[
Lets start with the Runes I have chosen. I chose armor penetration marks because i just love armor penetration in an attacking character. theres nothing like just blowing through a pesky tanks armor. I took health seals and health Quintessences, because with the combination of the health seals, quintessences and his enrage skill it isnt very necessary to buy health items. And lastly cool down reduction glyphs because you always want your cryptic gaze spell to cool down as fast as possible so you can stun everybody whether its for attacking purposes or escaping a tough situation.
This might not be a standard mastery build but i find it works great. If you find a better one that works it doesn't really matter switching them out. I have put 22 points in defense because I like that extra dodge chance and with the movement speed. The rest of my points go to offense so i can get that little extra boost with attack speed which will be very useful in the game.
I start out with buying some boots and health potions at first. Boots because they are always useful to run away or catch someone early game if they did not start with boots, and health potions to keep me in my lane in the beginning as much as possible. Next time i am forced to go back to heal I grab berserker's grieves to help me with attack speed, and if i have enough money ill get an emblem of valor to start me on starks fervor. I get starks fervor because this is mainly what this build be able to be a part DPS. It gives me great attack speed bonus and the life steal and health regeneration helps your survivability in the beginning before you get those tank items. I also find that the 20% life steal combined with your cannibalism ability can be great with up to 120% life steal! Next item i get is the guardian angel to help with armor and its nice to have that backup in case you happen to die. Next is Force of nature. I love this item because of all the health regeneration it offers it is really great. Next i would get if your game is long enough and you have enough money would be either tiamat or atma's impaler. They both help with attack damage greatly putting you over the 200 mark, and atmas will give you armor and crit chance while the tiamat will give you the extra health regeneration. Lastly if you are able I would get a thorn mail and/or a sunfire's cape because they are just great items.
I use this skill sequence because I love the extra attack damage and the health bonus enrage gets you from the beginning so i max that out as soon as possible while getting cryptic gaze. I also get cannibalism whenever possible for ganking possibilities. Lastly i get deaths caress because i am just not a huge fan of the ability. I find sion's other abilities much more useful but if it works for you get it!
The summoner spells I usually get are heal and clarity. Heal in case you get yourself into some sticky situations and need a way out.and calrity because i find myself always running out of mana when it is really necessary to just stun someone or activate cannibalism for the kill so I often use calrity.