This guide aims at building an effective tank/jungler/ganker, the core items are [Shurelya's Reverie] - [Frozen Mallet] - [Frozen Heart]. The rest of her slots can be filled with general tank gear depending on what your against.

Pros:
Excellent ganks.
Excellent crowd controlling effects.
Excellent tank.
Can jump over walls to chase enemies or to run away with her Arctic Assault "Q" ability.
Actually pretty good farming capability once your "W" Northern Winds is leveled up a bit you can clear a minnion wave in a split second with "Q" "W" "E" combo, this can be good for stopping pushes also.

Cons:
Not alot of mana until you get Frozen Heart or the component for it atleast, though Philosopher Stone helps.
Not very strong pre lvl 6.
Easy to counter jungle sejuani.

1. Arctic Assault:
This sweet spell is used for alot of different things, you use it in jungling, clearing minnion waves, to initiate a gank, a team fight, or to chase someone down, but it's also a great escape mechanism too, it's an very easy to use wall hopper, jump over dragon wall, or baron wall, you'll quickly lose count of how many times this will save your life.

2. Northern Winds:
This ability is your main damage source, it has a low cooldown, and the mana cost remains a constant 40 through out every rank. this will be used to jungle quicker, clear minnion waves, and hurt enemy champions so your allies or you will finish them off successfully.

3. Permafrost:
This bad *** ability is used in team fights and ganks, it does up to 70% slow at rank 5. the ability effects anyone who is under the frost effect in a big radius around you. this is great to use in combination with your ultimate, as soon as the stun from your ultimate ability wears off you can use this to follow up with a massive slow which is often devastating.

4. Glacial Prison:
This ability is the best thing Sejuani has to offer the team. Early game this is an extremely effective ganking tool while mid and late game it can be used in team fights for area of effect crowd control (stun). Follow it up with her "E" Permafrost and your team should be able to dish out considerable damage before the enemy is capable of reacting.

I believe one slowing ability can stack with 1 item slow, with deminishing returns. if anymore abilities or item effects slow the same target the highest single one of each category takes presidence. so frozen mallet will be slowing 40% while perma frost will be giving another 70% I don't remember the exact formula for the deminishing returns but this is an insane amount of slow anyway, which is why I called it Slow lock. maybe im biased but Frozen Mallet is one of my favorite items in the game. and Sejuani's passive is 10% slow frost which stacks with deminishing returns makeing noone able to run from her.