Pros:

• Amazing Teamfighting
• Great AoE CC
• Good Peel/Engage
• Ult can single handedly save your team or turn around a teamfight
• Infinitly Scaling HP (multiplied with CinderHulk)
• W grants an extra 400hp-600hp shield late game
• Brick wall late game, but decent ganks post lvl3
• Still usefull even when behind

Cons:

• Clears aren't overly fast
• All forms of CC (Q, E, R) CAN be missed
• When his abilities are on CD (or when he's in his passive) he is easily kited and doesn't serve anymore of a purpose besides absorbing a metric ****-ton of dmg

• Greater Mark of Attack Damage - Standard marks for an AD Tank. Armor Pen marks could be used, however i'd deem them as useless seeing as only half of your abilities deal physical dmg and you aren't building any damage with this build to scale with pen anyways.
• Greater Seal of Armor - Once again, fairly standard seals. Scaling armor could be used but I prefer flat seals for early clears and ganks (Besides you're going to be building a lot of armor anyway)
• Greater Glyph of Magic Resist
  Greater Glyph of Scaling Magic Resist - I run 3 flat and 6 scaling mr glyphs as a healthy medium; Firstly, you're usually going to be building armor or health first, so it's nice to passively gain a bit more mr for those mid-late game fights. However, that doesn't mean you aren't going to need any mr through-out the early-mid game. Between ganking lanes once you hit lvl 3 and possibly even skirmishing with an enemy AP jungler, you are going to need at least some early mr.

Since all the other items in this guide are more or less self explanatory as to why to get them (e.g Hunters machete start as a jungler) this section will be reserved for the options you have available for your post-core tank items, when and why to get each one.

Warmog's Armor

[*] Warmogs is usually one of your first tank items to get, assuming that you aren't rushing into defenses. The reason it's a great item on Sion is due to your Soul Furnace scaling with health, and also Warmogs passive, which helps you regen a lot of health when moving around the jungle in between skirmishes and ganks in the early-mid game.
The only time you wouldn't rush Warmogs after your core is if
a) There is already a fairly fed carry on the enemy team (e.g Cassio mid, Jinx bot, Fiora top).
b) If their team is over 4/5 ap or 4/5 ad in which then you would rush your primary Hp+Defense item
(so in the case of a fed adc, you would most likily rush a Randuins Omen, or a fed ap mid you'd get Banshee's vail).
c) They have one fed, or multiple champs on the enemy team that have HP shred/true dmg (Vayne adc w/ BotRK etc) in this case you'd go for hp+defense item or a purely defense item. (Thornmail, Frozen Heart)

Randuin's Omen

[*] The next item you should be looking at after your core is Randuins Omen. This is a great all around item, granting a lot of health + armor, and also 2 handy passives which add to your utility.
DO GET THIS ITEM IF:
[*] The enemy team has a fed AD champ (Fiora, Riven, Talon, Rengar etc.)
[*] The enemy team is mostly gonna be dealing Physical damage

DO NOT GET THIS ITEM (FIRST) IF