I was goofing around and decided to build slow on sejuani. this is the build that it resulted in. This is a very first guide and is really only recommended for goofing off, im not sure if this will work against a good team. It uses the slows from Rylai's and FM combined with Sejuani's passive to slow someone to the point were they are barely moving. Hit Sejuani's e and they stop for all intents of purposes.

This is just what i was running for my games. Any suggestions would be greatly appreciated.

I start boots and three health pots for early sustain in lane and for a little bit more mobility. Then Rylai's will give you decent AP and Health. The slow is quite useful, especially when you use her Q and it triggers two seperate slows. The slows do stack. Phage adds more slow and more health, making it harder for anyone to force you out of lane. And then you get Rageblade. Rageblade is one of the best items in the game for hybrid builds and this build is no exception. The attack speed and ap stacks allow you to do brutal damage, as well a giving a boost to your ad. At this point it should be impossible for people to escape unless they have multiple flashes and such (Ezreal and Ahri for example). Finish off Phage into frozen mallet for extra tankyness. If you are doing brutally well you could instead finish it into a TriForce, the sheen procs when you use her Q are quite amusing.

The end of the build is kinda up to you depending on how you are doing. I like to build the WotAs because of the spell vamp and it will help any other casters on your team. The Abyssal scepter gives AP and MR which increase damage output and tankyness. However I could potentially see subbing in LichBane if you didn't build TriForce earlier. Also, you could easily sub in Guardian Angel and B-Veil for extra tankyness as well.

I take Q first because it gives you a closer and an escape as well as triggering Sejuani's Passive. Taking W next allows for better minion farm in lane. I then proceed to grab another rank of Q to increase the damage done. I grab one level of permafrost at 4 then proceed to max q followed by W then E while picking up my ult at 6, 11, and 16. Permafrost is good but the additional slow from your items means the increase in levels isn't as important. Just Q followed by W can deal a hefty amount of damage and your ult is a great way to initiate a fight. I would almost always try to target their Carries with ult because it will enable your team to easily kill them, leading usually to a won team fight.

This is my first guide, I hoped you liked it. It started as a troll build but worked well enough that i thought i should share it. Oh Summoner Spells take some combo of Flash Teleport and Ignite depending on what you prefer.