Gain 10% CDR when you reach level 10. Excess CDR becomes AP or AD, Adaptive.
Patch: 8.2
WhyUnsealed Spellbook
The key to this build is lane pressure and roaming. Spellbook plus Cosmic Insight allows you to keep your Teleport on a short cooldown so you're able to shove lane and roam.
WhyPerfect Timing
Zhonya's and Zilean's [R] grant numerous chances to minimize positioning mistakes.
Don't upgrade the Stopwatch into a Zhonya's early unless you're playing into an assassin matchup and are dying a lot.
WhyMinion Dematerializer
Minion Dematerialzer plays a key role in being able to keep the lane pressure/wave clear up so you're able to leave lane and find ganks or fights with your jungler.
The bonus damage to minions allows you to clear waves in 2 bombs without stragglers being left behind to auto attack.
WhyCosmic Insight
With Zilean's only damage ability being [Q], he heavily relies on the bonus CDR. The extra bit of Summoner CDR also means more TP pressure and Flash safety.
WhyThe Ultimate Hat
At level 16 Zilean's [R] is already pretty short. With max Ultimate Hat stacks that puts you at a 24s cooldown [R] heading into the mid game.
With good awareness should you be massively reducing the amount of deaths your team takes.
WhyTranscendence
Transcendence allows you to go Morello's and Lucidity as your only 2 CDR items, while still hitting cap.
It also opens the opportunity to buy Zhonya's if need be, against difficult assassin matchups.
