Pros / Cons
Pros
-Good Jungler with great ganks
-Super CC
-Very Strong Chaser
-Very difficult for people to escape your wrath!
-That boar is ADORABLE!
Cons
-If you use Arctic ***ult at the wrong time, it could result in your death.
-Timing with skill set is hard to figure out
Skills / Applications
*Application* This is 
Sejuani's passive and one of the most important things she can do, utilizing this passive alongside her other abilities is the secret to dealing damage and making the best of your crowd control.
*Application* This ability is nice, because it gives Sejuani a good bit of utility and can be used in a good range of different situations. It is and excellent ganking tool, because upon hitting the target it deals Frost which means that both 
Permafrost and 
Northern Winds can be activated upon colliding with the target which will not only allow you to stay on the target while your dealing a significant bit of damage, but you can also utilize a massive amounts of crowd control. Another thing that this skill is good at is initiation in the team fight, if your teammates are behind you when you do this you can cause devastating crowd control effects in the middle of the fight! My favorite use of Arctic Assault is using it to escape sticky situations. This ability is great for avoiding being ganked and escaping teamfights, but an interesting feature of this ability is that just like 
Malphite's ultimate 
Unstoppable Force you can dash through walls which makes it almost impossible for enemies to chase you especially if you choose to take 
Flash. This move is also excellent for tower diving and i don't really think i need to explain how.
*Application* This ability is the main damage dealer in Sejuani's skill set. It scales a little off AP as well as a percentage of health, lucky for you those are the main focuses of this build. The damage that can be dealt using Northern Winds can be increased if you are dealing damage to enemy champions affected by Frost so make sure you take advantage of this.
*Application* This is a pretty interesting ability. It only damages people who are affected by Frost, this increases the slow for a set amount of time. The percentage that it slows by goes up by 10% each time it is leveled up so don't overlook doing so. This ability is also what gives Sejuani the majority of her crowd control power as the slow can be activated at anytime while an enemy is effected by Frost. This ability has a good bit of utility as well. Since you can activate it anytime while enemies are under frost you can use it in a couple of different ways. You can pop it directly after an initiate using Arctic Assault to keep the enemy(s) from getting out of your main source of damage Northern Winds, it makes for a great chasing tool if you can get a good hit in with your normal attack, it can be popped directly after the stun from 
Glacial Prison for a horrifying crowd control effect on the enemy team during team fight, or you can also pop it on an enemy who is low on health for a cheap kill.
*Application* Just by looking at this move you can probably tell how destructive this ultimate can be, it has the potential to ruin an entire team fight for the enemy team. This ability has the potential to turn entire team fights around favorably. So lets discuss how it can be properly utilized as such. There are two real ways to use this move: offensively and defensively so I'm going to spend a moment to discuss each thoroughly.
Offensively: There a few options when considering using this ability for offensive purposes. First it makes for a great set up if you want to use it to gank. It can be thrown from a bush to hit an unsuspecting enemy. This works best if there is a teammate present. It is also good when in a chasing situation you can snag them with it, Arctic Assault, if you have it, then pop Northern Winds and after the duration of the stun if necessary you also have the option of using Permafrost for the extra crowd control and damage. The best way to use this ability however, is to throw it into a crowd of enemies during either the beginning or the middle of the team fight it is very easy to hit 2, 3, or even 4 enemies with this ability thus giving your team a good amount of time to gain the advantage.
Defensively: Using this ability defensively can be somewhat unfavorable, as it is more useful when on the offensive, but is still necessary. It can be used to save your own life or that of a teammates if they are about dead and being chased by one or multiple enemy champions. Simply throw Glacial Prison into the crowd and you or your teammate can get away Scott free, or if you can manage it this strategy can make for a great counter gank or counter strike tool. When used with Arctic Assault and possibly Flash it can be used as a virtual get out of jail free card. Its also great for catching people under the tower who think they can get a tower diving kill.
Creeping / Jungling
Start with a 
Cloth Armor and 5 
Health Potion at Blue Golem, Wolves, Wraiths, Lizard Elder, and Double Golem, then back to wraiths. And at that point you can gank or keep triangling around from Double golem to wraiths to wolves, and if you killed blue first he will spawn shortly after the 7 min mark. But the goal should be lvl 6 at 7 mins or lvl 6 right after getting blue the second time.
Masteries
I like to do a 0/21/9 making sure to take
Juggernaut
and
Veteran's Scars
also making sure to proc damage reduction in the defensive tree, and
Runic Affinity
in the utility tree. so long as those two masteries are prioritized the rest is very susceptible to change.
Items
There is a lot with this build that is subject to change depending on not only your own team's composition but the enemy's as well. I would like to, however, point out the core items in this build that i would definitely recommend keeping. What I consider the core items of this build would be 
Aegis of the Legion, 
Rylai's Crystal Scepter, and 
Frozen Heart. Though the other items are pretty good, if they aren't your style know that they are subject for change. See Diversity of Tanking for other item suggestions.
Aegis of the Legion this item makes for a good starter for a few reasons. First off it is easily build from a Cloth Armor so it makes for an excellent first item simply from a jungler's perspective. You get the optimal start from a cloth & 5 in a jungle as well as a tank item early with a pretty nice support aura. This also allows a bit more focus on AP and health seeing as it gives a pretty nice bit of tank for early game.
Mercury's Treads or 
Ninja Tabi As the tank I highly suggest going with one of the two defensive boots according to the enemy team's composition.
Rod of Ages This item may seem a bit controversial and isn't really necessary but I will swear by it on Sejuani it grants a lot of health, mana, and AP which are all things that she needs. The health granted will not only allow her to be a bit more beefy but, alongside the AP, it will contribute to Northern Winds. The mana is also nice too, especially if you aren't fully reliant on blue aura.
Rylai's Crystal Scepter I find that the extra 15% slow for your abilities really is a nice thing to have. This item really just exaggerates the power of Sejuani's skill set so much. It just adds to the power of Sejuani's heavy crowd control through slow. Especially when used in combination. You can use the slow/extra slow from all of your abilities on a single or multiple target and it becomes literally impossible for them to get away, especially in team fight.
Frozen Heart This is, in my opinion, the single most important item Sejuani can have! It grants some cooldown which will allow Sejuani to do more with her heavy crowd control. This item also grants her another supportive aura that will make her even more of a detrimental force in the team fight setting.
Guardian Angel This item is a good finisher but not necessary, it simply makes Sejuani harder to kill with the aura as well as it grants both armor and magic res.
2,726
1
Build Guide
Discussion (1)
Thornmail This item can be very useful if you are up against very fed AD carries such as: 
Tryndamere, 
Caitlyn, 
Corki, and people of the like.
Sunfire Cape this item will allow you to do a little bit more damage as well as take more armor, this is nice because 
Locket of the Iron Solari this item grants a good bit of health and armor as well as grants a support aura which would do well with the support aura you should already have from Aegis of Legion, it also gives a shield active that will give your team just a little extra umph in team fight
Force of Nature This item grants a good amount of regen which is just nice to have, but mainly it is an excellent source of magic res and the movement speed works very well with 
Abyssal Scepter This item can be good if you get fed and want to keep the damage rolling. It grants a nice amount of magic res. as well as some AP. It is also useful for reducing enemy magic res. which will make them more susceptible to your damage.
Greater Mark of Attack Speed allow for a much needed faster jungle, as well as allow you to take advantage of 
Ghost if you don't like 
Ignite good for grabbing kills just out of reach.
Exhaust good for utilizing with 
Heal always a good decision!
Cleanse just in case you need that extra help with crowd control
Surge to be honest I'm not sure about this one. The extra attack speed and granted ability power would work for 
Promote Yes 
Clarity you really don't need this because you have blue at the start also this build suggests building mana so i cannot see this as being too too useful.
Revive never get this unless you are 
Karthus
Clairvoyance leave this to the supports.
