Hello everyone, I'm a challenger jungler in the EUW server who has reached it on 2 different accounts this season.
Despite playing mostly carry junglers, every now and then the tank duty calls and sejuani is my to go choice when this happens, which lead me to write this guide since it's one of my favourite junglers and I have approximately ~300k mastery points on it.

sejuani Sejuani is a utility tank in the jungle. She's far from being the best solo carry in soloq, but she's one of the best champions to play as a team or with a duo. She has many great sinergies(specially with melee champions due to her E passive), a very reliable engage tool, strong ganks and scales well into the mid-late game. She's fun to play and it's a great addition to everyone's champion pool who want to provide to their team the tools they need to win.

Code:

Pros → healthy clear → semi-fast clear → good ganks → scales well → can engage fights → She has a lot of crowd control → Sinergizes well with a lot of champions → you can afford to fall behind a bit in xp/gold

Cons → weak early game → vulnerable to invades → relies on teammates → it's not hard to counterpick → weak duelist

Fury of the North
Sejuani's passive does 2 things: the first one gives you armor and magic resistance when you're outside of combat and immunity to being slowed. This is very useful to when it comes to things like facechecking bushes to get vision or to iniate fights, since you'll have extra resistances at the beggining and immunity to a crowd control The 2nd part gives you extra damage when you atack someone that you freeze with your other abilities, very useful to clear jungle camps and to deal some extra damage. Arctic Assault
Her Q is a dash who will knock up enemies at the end of it for a small duration, very useful to charge at people when you're ganking but also to escape a bad situation, it has a very long cooldown so take your time to think about what you want to do with it. It can also be used with your flash to extend it's range in the same way as shen E for example. first you Q and then flash at the end of the animation Winter's Wrath
Sejuani's W is a skill with 2 parts. Both of them give you a stack of her E passive. The first part is basically just a knock up to creeps and monsters who leaves them alligned. The 2nd part is the most important one, because not only it applies a stack, but it deals the most damage and applies slow to champions if you hit it. This is the skill you max first so that you can clear jungle camps more efficiently. Permafrost
This is arguably the most important skill on her kit and what makes her such a strong teamfight champion. Everytime you or ANY allied melee champion atack something, it applies 1 stack of Frost, at 4 stacks you can activate this skill to stun the enemy, point and click stun. Then any atack or ability you use on a frosted target, will deal that extra passive damage we discussed earlier. Despite being her core skill, it's the last one you upgrade since leveling up only gives you extra damage, and you play sejuani for the utility. Glacial Prison In her ultimate, sejuani will throw a projectile that stuns enemies based on distance and create a slowing area round it that deals damage once it detonates. This is your best skill to initate fights and try to get picks in the fog of war, however when you're ganking you usually don't want to start with it, since this stun counts as Frost which means you won't be able to activate your E aftwerwards for a few seconds. But if you start with E stun (autoatack for passive) and then ulti, they'll get stunned for the E duration + the R duration and you can apply your passive damage twice.

Aftershock:Sejuani has a lot of crowd control so she can apply this rune with pretty much anything but her W. It gives you extra resistances and a bit of damage which is perfect for tanks. Font of Life:Demolish doesn't make a lot of sense since you're playing in lane to kill turrets and sejuani doesn't have shields to make use of shield bash so the logical choice is font of life to give some extra sustain to your teammates in fights and skirmishes. Conditioning: Second wind doesn't make sense here since you're clearing camps and not taking damage from champs to heal back a bit, and bone plating isn't very useful as a jungler aswell, so conditioning is the best choice to give you extra armor/mr. Unflinching: Ever since this rune got buffed it feels like a must have if you choose into this tree, you get so much tenacity that cc barely hits you. even thou sejuani is a tank, being able to move around is still important to make sure you can apply your CC to carries. Triumph: Very useful rune to give you a bit extra of gold and to help you get back some HP after participating in kills. since you stack a lot of HP, you'll heal quite a lot with this rune in fights. Legend: Alacrity: the biggest weakness of tank junglers sometimes can be their slow clear, the way to compensate this is by going alacrity to give you extra atack speed, it works really well with sejuani aswell due to her E, you'll reach 4 stacks faster

Sunfire Aegis In my opinion this is the best tank item for sejuani. Despite being more expensive, it provides more damage from it's passive and the tenacity from the mythic passive is really helpful aswell, you already have enough cc on your kit, so getting a little extra damage feels good when you're in fights.

Warmog's Armor This is the 2nd item you'll rush on sejuani jungle every single game. It has been buffed a lot lately and sejuani's abilities do damage based on her HP, which means the more you have, the more damage you'll deal, it's the perfect item for skirmishes aswell since you'll heal back to full hp after leaving combat.

Gargoyle Stoneplate There's no point in having HP if you don't have the armor/magic resistance to follow it up and vice-versa. This is the item to keep you alive when you jump into 5 people and activate it. you'll be ready to buy enough time to apply your CC and to be the frontline to your team.

Force of Nature There aren't many options for magic resistance items, but out of all them, this is the one that gives you the most MR, and also movement speed on top of that which is exactly what you need on sejuani. Spirit Visage is worthless since you don't have healing in your kit and Abyssal Mask is just not a very good item right now.

Thornmail Buy this item if you're vs champions who have a lot of healing in their kit like Kayn, Olaf, Irelia,etc..

Randuin's Omen This is the perfect item vs crit champions and to survive the ADCs in the late game, it gives you nice stats and a very useful active to slow them down and reduce their damage to you.

Frozen Heart This item has been buffed a lot lately, it's in a good spot and it's perfect for you vs champions who have a lot of atack speed or vs teams that are full AD.

Sejuani doesn't have the best early game but she can afford to choose where to start. Ideally you always want to play around the lanes that have melee champions to make sure you get good ganks early.
So you want to start on the opposite side of the lane you want to gank, if you want to gank topside because you have a Camille or something, make your botlane leash or if you want to gank your Leona botlane start topside.
Always clear at least 4 camps before ganking, this means you don't gank unless you have 16 cs, because then if you commit to a gank you can at least secure priority on that lane to fight the scuttle. if you gank before that you become vulnerable to being invaded and you always want to full clear the side of the jungle that you start on.
When you start the game you should always put a ward to cover the jungle entry of the side you want to start on to prevent late invades, then you recall and grab the red trinket to make sure that when you try to gank you're unspotted on a ward.
Try not to overcommit to ganks early and farm to your 6, with your ultimate your ganks' success rate should increase a lot and gives you the possibility to gank any lane aswell.

To be finished..