Ascension

The Ascension is the phase that takes place during Hard Mode and grants access to War Mode after you have defeated the 5 Guardians of Terraria. The guardians each wake up after a certain event is completed, and they are some of the hardest bosses in the game, only being beaten in power by some of the War Mode bosses. Each Guardian you defeat gives you a special shard which appears below your inventory, and does not act as a normal item, being where it cannot be dropped, moved, destroyed, or deleted and only apperas in the world you defeated the Guardian in. Each Guardian can only be defeated once per world, and while one Guardian is active, special mobs might spawn in your world, and special events may occur.

Cthulhu
This withered sea god starts to stir whenever you first complete the Lunar Event, and is announced with the message "The tides are shifting..." During this period ocean monsters and more common and more types of them will spawn, including special raids each day by Cthulhu Warriors. After five days, this message will appear "The ocean grows restless". The spawn rates for ocean animals will go down to normal, but near the ocean the special mobs will spawn at higher rates, such as Cthulhu Warriors and Warcrabs. Once this period is reached, at the beginning of every day, there will be a 35% chance for Cthulhu to awaken and attack you, announced with "Cthulhu has ascended!"