Ascension is crucial as it increases your hero's base stats and unlocks higher levels. You can find the materials needed for Ascension in the hero's details section. These materials include Crests and other items, which can be obtained through specific encounters on the Campaign Map.
Starting from level 40, you'll need special Materials for Ascension. These are less common and more challenging to acquire, so keep an eye out for them during your adventures. If you are a battle hardened commander, you can try to conquer the hard campaign map which yields even more Ascension Materials!