On the Shadow Era Wiki Sustain is defined as a game mechanic that was introduced in Shattered Fates. This mechanic is found on cards that would otherwise be rather powerful for their cost. This effect causes the controller of a card with a Sustain cost to have to pay a cost (health from their hero or shadow energy generally) as the cost for keeping that card on the field.
Aside from a cards original casting cost, sustain is an ongoing unpreventable cost that a players HERO must pay at the end of each of your turns. The sustain cost must be paid in full for the card to persist and the cost can not be reduced in any way. At the end of your turn, if the sustain cost can not be paid, the card enforcing the sustain cost will self destruct. If there are more than one card with the same sustain costs, they will be paid at the end of turn in Standard Resolution Order.
Now let’s take an in depth look at some of the current Sustain costs and the cards associated with those costs: