This is a new feature that can be accessed as you progress through the story. You will typically choose between two heroes/Fatecores and start what is known as an Eternal Bond.
Conditions for Eternal Bond:
Log in consecutively for 30 days and you will receive a Fated Hero or their corresponding Fatecore + the Fated Hero.
Each day, you will be granted a reward as ‘motivation’ to continue the bond. You can expect to receive 30 rewards for the 30 days, the 30th being the Fated hero itself.
You can only start a new bond if you complete or destroy the old one. If you destroy the old bond, you must wait for the Hero to show up again.
A bond can be recovered if ‘disrupted’ (a few days where you didn’t log in) for a cost of Xes.
Sometimes there's a glitch where the time where the game resets gets messed up (usually after maintenance). I suggest checking into the game a few times to make sure your Eternal Bonds progress doesn't get messed up.
If you break Eternal Bonds too many times, you can no longer get the hero and the amount of Xes fine is hefty.
I don't suggest doing Eternal Bonds constantly or you'll get burned out. Moderation is key.
Eternal Bonds: Completion
Once 29 days have gone by, on the 30th day, you will receive your hero. The reward can be collected via mail from Yorm and once you select [receive], the Fated hero is automatically summoned in the traditional way. You can find your Fated hero in [manage heroes].
Personally, I think this is an awesome feature (especially for f2p) and a great motivator to log into the game. It doesn’t cost any currency and you are guaranteed to get the Hero of choice.