Health stacking and map control.
Durability and crowd control
After immobilizing an enemy champion gain defenses and later deal a burst of adaptive damage around you.
Impairing the movement of an enemy champion marks them. Your allies heal when attacking champion's you've marked.
Gain 5 Armor. Whenever you heal for at least 20 health or from a consumable's effect, increase your Armor by 5% for 3 seconds.
Every 8 monsters or enemy minions that die near you permanently increases your max health by 0.2%.
+65 health and +9 AP or +5.4 AD, adaptive
After killing an enemy ward, a friendly Zombie Ward is raised in its place.
Unique champion takedowns grant permanent out of combat MS.
Like Courage of the Colossus, Sejuani can reliably proc it for the bonus defenses and extra bit of damage.
All 4 of Sejuani's abilities can easily proc Font of Life, and because you naturally stack health the on-hit heal adds up to be considerable for fights.
As a jungler, early game armor allows you to sustain longer clears while remaining healthy for fights.
Because you naturally itemize stacking a lot of health as Sejuani, Overgrowth doubles-down on that by providing a lot of bonus scaling health based on how much health you've built.
Sweeping Lens and Control Wards are the mainstay of a jungler. Why not benefit by getting extra vision for yourself when you go on a ward-clearing spree.
It's important for a jungler to be able to quickly react and get to a skirmish happening anywhere on the map. Relentless Hunter gives you the Movespeed Speed necessary to get there and win.