Rhaast's Q damage scales heavily with AD, and since his Q deals max HP damage, you end up dealing 60% or more with a single Q at full build, same with his ultimate. With the Red Kayn passive healing you depending on how much damage you deal, your Qs will be healing you for a hell of a lot, and your ult will almost always leave you at full hp at 2 or 3 items.

Can be played Top as well.

Eclipse gives lethality and armor pen for more damage, but also gives omnivamp and a shield which help us do what we want to do.

Youmuu's gives more damage and some extra movement speed , but it can be traded for items like Axiom Arc for more ults, Edge of Night for a spellshield, or Black Cleaver to deal with tank-heavy teams.

Death's Dance adds survivability and a burst of HP after every kill which is super helpful in tense fights.

Muramana gives us a ton of mana to make sure we dont run out mid-fight, as well as on-hit damage on your abilities and a lot of AD which give the max HP damage on your Q and R a huge spike.

Ravenous Hydra finishes off the build with a huge amount of AD and extra omnivamp, as well as more on-hit to your abilities.

Boots can be changed depending on enemy team comp, late-game you should sell boots for a defensive item such as Sterak's Gage, Maw of Malmortius, or Spirit Visage.

This build also gives a ton of CDR which helps you spam your abilities more and have more chances to outplay and outheal your opponents.