Ethereum Serenity

Serenity is the last phase and has one key principle - to switch the ethereum network from proof of work to proof of stake - essentially reducing the power consumption of the ethereum network. When this is going to take place and under what conditions is of course anyone's guess - although it is thought to be in the planning for the start of 2017.

[1] https://www.cryptocompare.com/coins/guides/the-ethereum-releases-of-frontier-homestead-metropolis-and-serenity/