Ivern in early game
In the early game, Ivern is one of the most potent clearing junglers. he can counterjungle pretty effectively, but you can find out more about this on Youtube. His early ganks are really good if you hit your Q, that's the most important part of his kit imo.
Ivern in mid game
Mid game is where Ivern is the weakest, up until he collects his core items which are Athene's holy grail, Redemption and Guardian Angel in every single game!
Ivern in late game
Late game is where you evolve into a supportish jungler, which means you have to sacrifice your damage in order to help your allies! this is very important