Zac

Jungle

Someone’s About To Get Hurt

Early-to-mid game skirmishing and snowballing

Resolve

Durability and crowd control

Aftershock

After immobilizing an enemy champion gain defenses and later deal a burst of adaptive damage around you.
Font of Life

Impairing the movement of an enemy champion marks them. Your allies heal when attacking champion's you've marked.
Conditioning

After 10 minutes gain 8 Armor and 8 Magic Resist and increase your total Armor and Magic Resist by 5%.
Revitalize

Heals and shields are 5% stronger and increased by an additional 10% on targets below 40% health.

Inspiration

30-270 health (at levels 1-18)

Magical Footwear

You get free boots at 10 min. but you cannot buy boots before then. These boots give additional MS. For each takedown you acquire the boots 30s sooner.
Cosmic Insight

+5% CDR
+5% Max CDR
+5% Summoner Spell CDR
+5% Item CDR

WhyAftershock

Aftershock is the go-to rune on any engage tank. After jumping in with [E], the bonus Armor and MR allows your to survive the engage while sticking to your target and CCing them.

WhyFont of Life

Zac's [Q], [E], and [R] all proc Font on multiple targets allow you to prove massive healing for your entire team in teamfights.

WhyConditioning

Conditioning scales well with Aftershock, as both of them buff each other. The bonus resist from Aftershock make Conditioning stronger, and Conditioning resists after 10mins make Aftershock stronger.

All of these bonus resists scale incredibly well with Zac's massive self-healing.

WhyRevitalize

Zac does a lot of natural self healing when he pick ups his bobs. Revitalize empowers your jungle clear with a much healthier first clear.

It also allows you to stay alive in fights if you're effectively picking those blobs up, especially at low health.

WhyMagical Footwear

Magical footwear's raw gold efficiency allows you to hit your 2 item power-spike just in time for mid-game teamfights.

WhyCosmic Insight

Cosmic Insight's overall effectiveness enhances your teamfighting with the extra CDR as well as granting you a few extra Smites with the summoner CDR.