Hello, this is my first guide for mobafire. I've had some experience with Sejuani and I've seen builds thrown around, so I thought I would put my own guide up here. Sejuani is a tank that has good initiation abilities while being able to do a decent amount of damage. She works really well as a jungler thanks to how quickly she can clear and her Q combined with her E makes ganking work really well. This guide focuses on building her as a straight up tank.
I utilize the 0/21/9 build in order to get the defensive materies that help with jungling (Tough Skin and Bladed armor) along with extra health to benefit her W (2% of max health at level 5) while taking reduced damage. The 9 in the utility tree is meant to help with Sejuani's early game mana problems (once your blue runs out), to improve flash, and to increase the red/blue buff duration.
I use armor pen reds because all of her abilities damage is counted as magic damage, flat armor yellows for the early game bonus and help jungling, MR per level blues because they overtake flat MR runes at level 10 and before then MR isn't a problem, and flat AP quints because all of her abilities benefit from AP. You can also use HP quints if you wish.
The starting items are fairly standard for a jungle build. Cloth armor and 5 pots in order to be able to clear the jungle in one sweep and safely. I recommend saving up at least enough to get Madreds Razor + Boots on your first trip back. After that, turn the Madreds into a Wriggles Lantern in order to get lifesteal, a free ward, armor, and some additional damage.
After you get the Wriggles Lantern and Mercury Treads, you should work towards a Warmogs in order to get additional health for more survivability + more damage on your W. After the Warmogs, your next item is dependent on what the enemy's team composition is and who is doing the most damage. If it's an AP caster, grab a Negatron Cloak. If it's a Brusier/AD Carry, group a Chain Vest. Once you grab one of those, grab the other.
I usually turn the Chain Vest into a Thornmail after that due to the amount of damage an AD carry will start to accumulate. You can replace the Thornmail with a Frozen Heart depending on what the enemy team has.
After the Thornmail or Frozen Heart, a Banshee's Veil is required for the additional HP + MR + spell shield in order to lower AP Caster's damage/burst.
After the Banshee's Veil, a Guardian Angel helps because of the additional armor, MR, and rebirth that it gives. This helps you stay alive longer in fights and rebirths you upon dying, causing you to be able attempt to start fighting once you rebirth or get away (because her Q can go over walls).
After the Guardian Angel, I grab a Force of Nature for the extra MR, HP Regen, and Movement speed. With the increased MR, AP casters will be even more useless to stop you and with the increased movement speed nobody will be able to escape thanks to the 70% slow that your E and the 10% slow your passive does.
I get W first for jungling because of the AOE it does, relatively short CD (compared to her other spells), and the amount of damage it does. After that, getting her Q can set her up for either a level 2 gank or set her up to clear the jungle faster. She can clear the jungle faster with her Q because when she attacks an enemy with it, it applies her Frost passive to the enemy. Once an enemy is frosted, her W does double the damage to it. After getting her Q, I get E next because it makes it almost impossible for people to escape ganks unless they flash. This is also the first ability to completely max due to the 70% slow it provides at max level. Using her E also lets her kill all of the creeps in the jungle within 1 combo with a few auto attacks thrown in. After maxing E, I max W next due to the increased damage it does and how increasing her W increases the HP scaling on it (Starts at 1% at level 1 and ends at 2% at level 5). Q is last due to it's lower damage and relatively long CD it has, making it not so good for clearing the jungle.
When initiating a fight, it's best to Q in, immediately E then W. Try to aim for the AD carries/ap casters in order to prevent them from escaping. Your ultimate is also a great tool for initiating, or to help escape.
I use Flash and Smite on Sejuani whenever I play her. Using Smite on a jungler is pretty self explanatory, and using Flash helps you escape combat or helps you gank and initiate fights.
Creeping / Jungling
When I jungle with Sejuani, I start at blue with a leash. Smite is required in order to kill blue. After I have blue, I clear the Wolves, Wraiths, and Golems. After this, smite is usually off of CD. I then take on red, and start to look at lanes determining whether any lanes are gankable or not. If they are, I attempt to gank them. If not, I continue roaming the jungle or attempt to counter jungle. Once you have your Wriggles Lantern, you should constantly ward either Dragon or your solo Top lane's river bush. This is to notify you whenever they attempt to dragon, allow bottom lane to know if mid is coming down or vice versa, or to let your top lane know if he may get ganked soon.
Sejuani is a powerful tank, strong jungler, and a great initiator. This is the build that I've had the most success with over the several games I've used it in. Let me know if you believe there is any room for improvement or anything you believe should be edited/changed. Feedback is appreciated.