in the rebirth version i don't know but in the original roshan defense map you had to play as pangolier or control and sacrifice in the pit a faction unit, those who appear after killing a map boss, you have 15 seconds to feed one of them 50 bananas to make it a permanent unit or it dissapears, after that a gem of true sight is dropped and you have to find 9 items letter which were R O S H A N D E F, those are dropped by invisible frogs around the map then a new ritual item is generated at the end a frog that add 1000 magic damage to auto attacks is created, see this as guide https://youtu.be/jmAfezG3zKY?t=2613