Yo w***up,im from Portugal i have 17 yo, im currently a plat hardstuck player i reached my peak this season it was plat2 with lee sin, cant carry this bonobos these days, ive been playing since season 3.
About this build, it is just amazing, it provides alot of damage and cc, you can easily 1v1 the enemy adc with ignite up, its a 5 sec cc if applied well combo.

+Practically cant die in a fight because of ultimate damage reduction
+Insane amounts of CC
+Really good Engage
+Has a nice amount of sustain with passive

Alistar is simply good, try it to see it.

Cons
-If you mess up an engage it can be punished easily by the enemy
-His main engage combo(W Q)can be dodged by mobility and canceled by hard CC
-Alistar has very long cooldowns on his abilities.

Flash-Flash is a ground-targeted summoner spell that causes your champion to blink a short distance in the direction of the cursor. It allows you to outplay the enemies. Use it in lane when you are under turret, the combo is Q-Flash-E-AA-W
Ignite-Ignite is a unit-targeted summoner spell that deals true damage over the duration and applies healing reduction as well as granting sight of the targeted enemy champion. It serves great usefullness fighting 1v1 after everfrost, allows you to takedown enemies with great hp gaining.

Phase Rush-Damaging basic attacks, abilities, item effects and summoner spells generate stacks on enemy Champion icon.png champion hit, up to one per attack or cast per champion. Applying 3 stacks to a target within a 4 second period grants you 40% − 60% (based on level) Movement speed icon.png movement speed and 75% Slow immune icon.png slow resist for 3 seconds.
Nimbus cloak- Casting a summoner spell grants Ghost.png ghosting and 5% − 35% (based on summoner spell cooldown) Movement speed icon.png bonus movement speed for 2 seconds.
Celerity- All Movement speed icon.png movement speed bonuses are 7% more effective on you.
Additionally, you gain 1% bonus movement speed.
Waterwalking-While in the river, you gain 25 Movement speed icon.png bonus movement speed and 3 − 18 (based on level) bonus Attack Damage or 5 − 30 (based on level) Ability Power (Adaptive). This allows you to roam midlane.
Hextech Flashtraption- While Flash Flash is on cooldown, it is replaced by Hexflash Hexflash.
ACTIVE: You begin Channeling icon.png charging for up to 2.5 seconds, increasing Hexflash's range over the duration. After at least 1 second, you can reactivate the spell, which is automatically done so when the charge completes without reactivation. Releasing the charge earlier than 1 second or entering combat with an enemy Champion icon.png champion will put Hexflash on Cooldown reduction icon.png cooldown for 10 seconds.
RECAST: You Flash.png blink to the target location within range. Carefull dont forget about the cooldown, it may cause you to use flash instead of hextech.
Cosmic Insight- Gain 18 summoner spell haste and 10 item haste. Good for everything, flash, ignite, everfrost.

Alistar is a top tier in the botlane right now with the normal build, imagine with this one, it makes you climb like a challenger even tho you are dog sheet. I hope you enjoy it as much as i do.

If you want to watch my stream my twitch is SpinningHeadKick