One of the 5 Darkin that was sealed by a child-sized demigod into a scythe. Currently wielded by Kayn in an attempt to prove that he is now superior to Zed. If he fails, Rhaast will overtake his body and relive his former glory before causing untold carnage upon his return.
Most people end up picking Rhaast in League of Legends because they got sick of the edgy boy and would rather listen to the yelling man.
Rhaast is the Doomguy of League of Legends. Only rip and tear until it is done.