I am also interested in knowing why sometimes they buy Essence Reaver and sometimes they replace it with another lethality item?

@@gabrielgonzalez1831 so basically Trinity force (TF) and manamune are for mid lane corki, you don't have a support to protect you so you want the health of TF and you cast a lot of abilities with multiple rockets on R so you benefit from the spellblade passive of TF, manamune for mana issues let's just call it a safe scaling build as you go critical after that. Lethality critical build is more for bot lane corki you have a support tank or mage who can protect you so you go full damage no health. Essence Reaver is a great item if you find yourself out of mana fast ( passive mana on hit ) if you don't have mana issues start Collector.